Introduction: The heat-treated product is an aquatic material of a high quality that can be used alone or mixed with other components to prepare different products with different flavors and tissues. Since the most expensive ingredient in this is a meat product there is the possibility of cheating or replacing less valuable tissues. Materials & Methods: In the present study, to detect unauthorized tissue in heated samples of fish meat, 30 samples were purchased of food stores, and then in a cold condition, the tissue was transported to the laboratory. After preparing five mm samples, they were placed in 10% formalin solution for at least 48 hours.Then, the usual steps of laboratory slide preparation including dehydrating with ethyl alcohol, clarifying with xylol, impregnating with paraffin, molding, cutting with a microtome to a thickness of 4-6 microns and staining with hematoxylin-eosin were performed. To study the unauthorized tissues in 36 microscopy lams, they were subjected to a precision microscopic examination according to the national standard related to the characteristics of the product. Results:The results showed some tissues such as tendon, soy, salivary gland, smooth muscle, clear cartilage, skin, fat, intestine, and scales in the product. Conclusion:Therefore, among the methods used, microscopic study can be used as one of the common methods to detect the presence of unauthorized tissues in processed products.