Pottery is one of the most common Iranian arts that has always been the manifestation of various decorations by various designs. Themes in the art of pottery express themes and concepts, which ceramists have used as a platform to express their thoughts, beliefs, culture, etc. One of these themes that is common in pottery of every period and has been used many times is the theme of fish. Fish is one of the original themes that has been used in Iranian art before and after Islam as a decorative theme and according to the beliefs of the people works as a symbolic theme in various arts such as carpets, pottery and textiles, etc. By analyzing each of these valuable artistic elements, one can understand the culture, civilization, mentality, etc. of these people. Although, this role may have changed during various times and periods due to different cultural, religious, social, etc. reasons, and sometimes with a natural form and structure, and sometimes in an abstract form. Nevertheless, it has not lost its main place and has always been used as a main and secondary them in decorating works (textiles, metals, carpets, pottery, etc.). Pottery is one of the arts that is often decorated with fish. In general, a large number of pottery from the Islamic period of Iran are painted with fish. In some of these potteries, the role of fish is the main subject of the work and in some others it plays a role as complementary theme. In both cases, this theme appears in various forms, structures, compositions and has created a beautiful reflection on these works. In this study, we try to examine the fish theme visually (composition, structure and form) according to the existing images, which leads us to the introduction and recognition of fish theme in dishes and understanding of types of composition, structure, form, etc. Now these are the questions What is the role of fish in Iranian pottery in terms of composition and structure? What is the appearance and position of fish in these potteries? This research has been formed with a descriptive-analytical approach and with reference to library and internet resources. By non-probabilistic sampling method, 30 samples of pottery with fish motifs belonging to the 6th to 14th centuries AH  were elected from foreign museums and collections such as Lakma Museum, Boston Museum of Art, Brooklyn, Harvard Art, Victoria and Albert London, Louvre, David Collection and Islamic art books, Then, in tables with linear examples, the types of composition, their structure and form are studied and the results are given so their properties can be analyzed.    The results of this study show that the fish theme is very rich and widely used in different forms and designs, which shows its visual power and high value. Spatialization of the fish design is so that it sometimes can be used as a decorative element in the form of a single theme or sometimes with other themes in different locations of the picture (center, corner, whole,…) depending on artist’s purpose. According to the research and analysis based on the existing samples of pottery with fish theme from the 6th to 14th century AH, they can be divided into five types of compositions. The fish theme is depicted in five types of index compositions (1/4, circular, horizontal, spiral, vertical).1.The 1/4 composition: is seen with a few fishes often on the corner and the bottom of the image. 2.The circular composition: in the center, whole or part of the image with the number of two or more fish is displayed. 3.The horizontal composition: The fish themes along with other themes (the subject of Iranian old stories) at the bottom of the second half of the image are shown rotating from left to right. 4.Spiral composition: Covering the entire space, they are seen moving and spinning on the spiral arch from inside to outside or vice versa. 5.Vertical composition: It is displayed linearly and statically.The obtained structure shows that no division has been made about this theme, and this theme has the ability to place and spatialization in different locations of the image with various number. The form used in most samples is close to reality and its lightness, softness, flexibility, and movement direction is used to move with or without the appearance (scales, wings, mouth, eyes) and is completely abstracted in various forms. Everything indicates that there is no limit to the drawing, numbering, and placement of this theme in the natural or abstract state in all periods..