Abstract: As a new type of antiparasitic drugs, ivermectin (IVM) has been widely applied in agriculture, stock raising and aquaculture in China because of its broad spectrum and high efficiency. In order to evaluate the IVM’s reproductive toxicity to male Crucian carp (Carassius auratus), IVM was orally given to the experimental fish with different dosages and the gonadosomatic index (GSI), sexual hormone contents (including testosterone and estradiol) in serum and testis, γaminobutyric acid content in serum and brain tissues, ultrastructure of spermatozoa and gonadal tissue in fish were determined in this study. The experimental fish were classified into A, B, C and D groups corresponding to the different dosages of IVM (0, 0.3, 0.9 and 1.5 mg·kg^-1, respectively once a day for 3 days continuously). Several indices in fish were detected after 8 days selfpurification. The results indicated that GSI gradually decreased with the increase of drug dosage, and GSI in groups C and D was significantly lower than that in group A. The contents of testosterone, estradiol and γ-aminobutyric acid exhibited a trend of first increasing and then decreasing and reached the peak at group B. Sperm longevity gradually decreased and the motion time also decreased in Ⅱ, Ⅲ and Ⅳ level sperms with the increasing dosage of IVM, which appeared to be especially obvious in group C and D. No obvious differences were found in the ultrastructure of spermatozoa and gonadal tissues. In conclusion, this study suggested that IVM had no obvious reproductive toxicity to male Crucian carp at the normal therapeutic dosage but could cause serious potential reproductive toxicity to fish at a high concentration.