Today in aquaculture to prevent negative effects of antibiotics The use of alternative, such as probiotics, prebiotics, organic acids, That can have positive effects on fish physiology activities. This study in order to done the synergistic effects of organic acid (potassium sorbate) and probiotic (Lactobacillus casei) in rainbow trout fry (Oncorhynchus mykiss). For this purpose of 360 pieces of rainbow trout with an initial weight (2.45±0.05), at 6 treatments with 3 replication include: Commercial diet (control), diet 10⁷ CFU/g probiotic (Lactobacillus casei) (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 probiotic (L. casei) (treatment 4) and the composition of the diet containing %1 potassium sorbate and 10⁷ CFU/g probiotic (L. casei) (treatment 5) was carried out. Feeding in two stages and trial period of 60 days. Sampling of fish To search Digestive enzymes, intestinal Histomorphology and juveniles resistance against pathogenic bacteria (Yersinia ruckeri), was conducted At the end of the experiment. The results showed, The activity of digestive enzymes was significant in the different treatments (P<0.05). Also Survey intestinal Histomorphology showed no significant difference In different treatments (P>0.05). Results of the test resistance to pathogenic bacteria (Y. Ruckeri) showed That mortality rate during the 14 days was significant among treatments (P<0.05). In total, according to the results of this study, suggested Add, combine %0/5  potassium sorbate and 10⁷ CFU / g probiotic (L. casei) in diets for rainbow trout.