This study focused on investigating the synergistic effects of potassium sorbate and Lactobacillus casei in rainbow trout fry (Oncorhynchus mykiss) with the initial weight of 2.45±0.05.A completely random designed  experiment was carried out for 8 weeks using six treatments, including: commercial diet (control), diet 10⁷ CFU/ g bacteria (treatment 1), a diet containing 0.5% of potassium sorbate (treatment 2), a diet containing 1% potassium sorbate (treatment 3), the composition of the diet with %0/5 of potassium sorbate and 10⁷ CFU/ g Lactobacillus casei (treatment 4), and the composition of the diet with 1% potassium sorbate and 10⁷ CFU/ g Lactobacillus casei (treatment 5).The results showed that the highest final weight, SGR, FCR and PER were observed in the treatment 4 which revealed significant differences with other treatments (P<0.05). However, no significant differences were observed between the different treatments regarding the carcass structure (P>0/05). WBC was significantly higher (P<0.05) in T4 compared with the other groups. No significant difference in the amount of RBC, Hb, HCT, MCV, MCH, MCHC were observed (P>0/05). In addition, the number of intestinal bacteria and lactobacillus showed significant differences among the different treatments. and highest rate was observed in treatment 4 (P<0.05) In total, according to the results and the positive observed effects, the addition compound CFU/ g 107 bacteria (Lactobacillus casei) and 0/5% of potassium sorbate in the diet of rainbow trout diet is recommended.