Background and aim: The most common and the best way of preventing microbial decay of marine foods is using freezing technology and the cycles and methods of defrosting have considerable effect on microbial changes of frozen shrimp. Shrimp is one of the marine foods that due to high active water (aw) and neutral PH and autolytic enzymes have high decay. Thus, in this study the effect of various methods of defrosts on microbial contamination of shrimp was investigated. This study was an empirical design on Penaeus merguiensis. The shrimps were divided into three groups including 1- peeled and headless (PUD), 2- Complete, 3- Headless (with skin) being frosted and defrosted in three cycles. Each group of shrimp was classified in terms of the type of defrosting method in three groups as 1- Microwave, 2- Refrigerator, 3- Water and were investigated in 3 cycles with the interval of 4 days. In this investigation, the total bacteria, Psychrophil bacteria, coliforms and Staphylococcus aureus were counted in private culture mediums. For data analysis, repeated measure Anova was used. All the bacteria including Psychrophil bacteria, coliforms and Staphylococcus aureus during the cycles had significant reduction process and this reduction showed significant reduction in complete shrimp and defrosting with refrigerator compared to other groups (P<0.05). According to the results, complete shrimp was the best kind of shrimp in terms of microbial load. Thus, avoiding temperature changes during transportation and avoiding unduly defrosts in maintaining the quality of the frozen shrimp is proposed.