A study on the mass changes of physical features, proximate composition and fillet yield of four red tilapia groups including 300-400 g, 400-600 g, 600-800 g and larger than 800 g was carried out with the aim to determine the optimal weight for fillet products. The results of the study showed that, a remarkable difference in physico-chemical parameters among the studied groups was found (p0.05). The significantly highest fillet yields were found in two groups of 600-800 g and >800 g if compared to that of the remained groups. Moreover, there was no significant difference of protein, lipid, ash content (in wet basis) between the group of 600-800 g and the group of larger than 800 g (p>0.05) except moisture content. In conclusion, the group of 600-800 g was the optimal weight for fillet products.