This research was conducted to determine optimum male breeder age and effects of the merely male breeder replacement (MBR) at different ages on reproductive performance in Japanese quails (Coturnix coturnix japonica). In control group no MBR was carried out until 28 weeks of age. And in second, third and fourth groups, males were replaced with 8 weeks old younger ones at 16, 20 and 24 weeks of age, respectively. At these MBR times, randomly collected hatching eggs from groups mated 1:3 (male to female) ratio were artificially incubated. Then hatching results such as fertility, early and late embryo mortalities, and hatchability of total and fertile eggs were compared. In conclusion, a sharp decline in fertility by male aging was observed and statistically significant differences in fertility between MBR groups occurred. Maximum fertility was obtained with 12 weeks old males. Fertility was above 90 % up to 20 weeks of age.