Objective: Even though boron (B), as a trace micronutrient, occurs in natural waters and organisms, its high concentration could cause harmful and even toxic for organisms. The aim of present study was to investigate the effects of boric acid (BA) added to feed (0.01%, 0.05%, 0.10%, and 0.20% of B in feed) on Rainbow Trout (Oncorhynchus mykiss) by histopathological methods and compared to the control feed as without B. Methodology: At the end of the feeding, after weighing the live weight of 9 rainbow trout from each group, tissue fragments were taken from the liver, gill, kidney, skeletal muscle, spleen and brains of fish, which systemic necropsies was done, and were fixed in 10 % buffered formalin solution. Results: Histopathological examinations revealed degenerative and necrotic changes in the liver, gill, kidney, skeletal muscle, spleen and brain. Hydropic and vacuolar degenerations in liver parenchyma, lamellar edema in the gills, hyaline accumulation in the kidneys, degenerations in the muscles, necrosis in the spleen and hyperemia in the brain were observed in all groups except control group. It was observed that the destruction of boric acid on fish increased depending on the amount of boron supplemented to the feed, and the most effect was in the group fed with 0.20 % boron supplemented feed. Conclusions: The findings aside from causing pathological changes in all organs in terms of histopathological findings of Boric Acid (BA), the most severe lesions were observed in the liver.