Introduction: The rainbow trout is considered the most abundant farming fish after Carp and has been broadly dispersed throughout the world. Compared to other Salmonids, this fish is capable of tolerating a wider range of water temperature and environmental variants including water quality. Mass production of this fish precipitates a decline in water quality and high degrees of Esterification, which consequently leads to emergence of various Infectious diseases among fishes. Apple cider vinegar, made from fermented apple juice, plays a practical role in eliminating hazardous bacteria as well as neutralizing and removing toxins of body. Owing to its antibacterial function, it is effective in boosting immunity and health improvement. The major objective of this study is to compare impacts of applying different stages of Apple cider vinegar in the Rainbow Trout diet, on various morphology indicators and the length of intestinal villi in the fishes with Non-alcoholic fatty liver disease. Materials and Methods: 120 pieces of Rainbow Trout which are experimentally afflicted with Non-alcoholic fatty liver disease, have underwent 4 diet treatments with stage 0, 1, 2, and 4; each is repeated 3 times, and each repeat included 10 samples in distinctive pans during 56 days. In order to sample, 3 fishes from each repeat were chosen, and anesthetized using Clove powder (0.5 gram per liter). After sampling, 3 to 4 centimeters of middle one-third of fish’s intestine was cut and placed in 15cc falcons containing 10% Neutral buffered formalin. Olympus Bx41optical microscope and Olysia zoom 3.2 soft imaging system were utilized to examine morphology of intestine samples. Average treatment comparison was done according to Tukey test significant 5% (P<0.05) by SAS statistical software version 9.4. Results and discussion: Results derived from statistical analysis of observations, demonstrated that there was no noticeable difference in height and width of intestinal villi, muscle thickness, Submucosa and crypt thickness between different treatments. Conclusion: different levels of apple cider vinegar in each dietary treatment had no significant effect on the studied intestinal morphology indices of rainbow trout.

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