Cadmium is one of heavy metals that naturally are present in crude oil and its derivatives. For the purpose of this study, a total of 120 Carassius Auratus with the average weight of 1.5-2.5 gr. were collected from Goldfish Reproduction and Culture Center in Golestan province, Iran. This study had 5 treatments and 3 repeats for each treatment in dosage of 0, 2.8, 5.6, 8.4, 11, 2 ppm that were 0 (control), 25%, 50%, 75% and 100% of cadmium LC50. At the end of the seventh day one fish of each repeat and lam were sampled for histological study. The results of tissue studies showed mucus, secondary lamella fusion, epithelial lifting, clubbing, shortening of secondary lamella, chloride cell hyperplasia, lamella degeneration and thinning of lamellae. The overall result show that gill tissue was infected during one week exposure to Cd with the increase of exposure time and the amount of poison and that the highest amount of infection was observed in the high poison rate at the end of the week. The findings could be used to evaluate histopathological effects of heavy metals on natural ecosystems within experimental conditional laboratory.