Abstract: Objective To explore the relation between semen parameters and maturation of sperm nucleus and spontaneous infertility in male Fisher344 rats. Methods Thirteen months old male Fisher344 rats (group A4) were studied comparatively with other group rats on semen parameters and sperma to genic cells and maturation of sperm nucleus. The other group rats were 4，7，10 months old male Fisher344 rats (group A₁, A₂, A₃) and 4，7，10，13 months old male S prague-Dawley rats (group B₁, B₂, B₃, B₄) and Wistar rats (group C₁, C₂,C₃,Ca). Results The percentage of sperm viability of group A4 was lower significantly than that of the other groups (P＜0.01) except group Ai (P≥0.05).The percentage of abnormal morphological sperm of group A4 was more significantly than that of the others (P＜0.01 or P＜0.05).The percentage of “a” and “b” grade motility of sperm was lover significantly than that of the other groups (P＜0.01). The percentage of Aniline blue strongly positive in sperm nucleus of group A4 was more significantly than that of the others (P＜0.01). The percentage of Aniline blue negative in sperm nucleus of group A4 was lower significantly than that of the others (P＜0.01). Conclusions Spontaneous infertility or decrease of fertility in male Fisher344 rats maybe due to the lower sperm viability, motility and maturation of sperm nucleus，and more percentage of abnormal morphological sperm.