This study aimed to investigate erosion of various fins in different weight groups of rainbow trout Oncorhynchus mykiss. Weight, standard length (SL), length of pectoral, ventral, dorsal, anal and caudal fins in four weight groups of fish including 20, 50, 100 and 400 g were measured and relative fin length to SL were determined for all fins. The results showed significant differences for all weight groups in most relative indexes exclude relative right and left pectoral fins to SL. The highest significant relationship of fins length with weight was observed in fish weight of50 and 100 g, while there was no relationship in those measured parameters in weight group of 400 g. The results of this study demonstrated that higher sensitivity of fin erosion in fish weight of 50 g, but the highest fin erosion was observed in higher weight groups which revealed the positive relationship of fin erosion with fish weight. So that, this index could be a useful indicator for evaluation of fish welfare and quality in rearing condition.