Current research aims to study the biological effect of mushroom consumption on body weight gain , internal organ weights and biochemical analysis of obese male rats. Forty male albino rats were randomly divided into two main groups. The first main group (8 rats) was considered as negative control group (healthy rats) fed on basal diet while the second main group (32 rats) were induced with obesity by feeding rats on high animal fat diet , the second main group consists of 4 subgroups each of (8 rats).One of these groups was chosen as a positive control. The rats in the positive control continued feeding on high animal fat diet. The three remaining groups of rats received high animal fat diets enriched with different levels of dried oyster  mushroom "Pleurotus ostruatus" (6%,9% and 12% mushroom) for 6 weeks. The results revealed that all obese groups which feed  with 6%,9% and12% mushroom resulted a decrease  in body weight gain ,on the other hand internal organ weights data declared that there are a significant differences (p<0.05) between  the positive control group  and obese group which treated with 12% mushroom in liver and kidney weights, whereas the results showed that there are significant differences (p<0.05) between  the positive control group  and obese groups treated with 9%,12% mushroom for heart weights. The results indicated that all obese groups which treated with 6%,9% and 12% mushroom resulted in significant decrease (p<0.05) in the values of serum cholesterol, TG , LDL-c  and VLDL-c but showed a significant increase (p<0.05) in the values of serum HDL-c comparing with control positive group .For liver enzymes, results showed  decrease in GOT, GPT and ALP in obese groups treated with 6%, 9% and 12% mushroom. The results also showed that there were no significant differences between the positive control group   and obese groups which treated with 6%, 9% and 12% mushroom with respect to kidney functions (urea, uric acid and creatinin). The study recommended that the addition of mushroom to diet to overcome the problem of obesity beside improving   the health of infants and adults.