In This study that was accomplished to investigate the relationship between levels of white blood cells (WBC) and gill tissue lesions in Flower horn, at first the number of 216 Flower horn fishes with average weight of (0/95 0.2) g and average length of (3.7 0.2) mm in 6 treatments, each treatment with three replication for 100 days with diets containing different levels of cow skin hydrolyze were fed. Treatment (T1) containing 20%, (T2) containing 40%, (T3) containing 60%, (T4) containing 80%, (T5) containing 100% replacement of cow skin hydrolyze instead of fish meal and control diet (T6) had no cow skin hydrolyze. After 100 days the gill samples of each treatments, was excised randomly (each treatment 9 gill speciments) and obtained samples were bleeded. then from gill of that bleeded fishes, tissue samples were prepared. The results of comparison between the levels of WBC of these fishes and assessment of gill tissue samples showed that in the treatments (T4) and (T5) that had more hydrolyzed powder in its diets, in comparison to other treatments, along with an overall reduction in the level of WBC and percent of lymphocytes, had more lesions in its gills.