To evaluate the adequate therapeutic dosage of clomipramine 15 mg/day and clomipramine 30 mg/day in male patients with premature ejaculation (PE), this study enrolled men aged 20–65 years who met diagnostic criteria for PE including Intravaginal Ejaculation Latency Time (IELT) less than 2 min for at least 75% of their sexual intercourses. Subjects received placebo, clomipramine 15 mg, or clomipramine 30 mg prn (2~6 h before intercourse) for 4 weeks. Efficacy was assessed using fold change, percentile change, and mean change of IELT, as well as Drug Coitus Interval Time (DCIT). A total of 101 patients were randomized into the placebo group, clomipramine 15 mg group, and clomipramine 30 mg group. Analyses of fold changes of IELT in each group revealed that the IELT of both the clomipramine 15 mg group and clomipramine 30 mg group was significantly increased 4 weeks after administration than the placebo group. Adverse events were reported by 11.76, 32.35, and 57.57% of patients in the placebo group, clomipramine 15 mg group, and clomipramine 30 mg group, respectively. Most common adverse events in the clomipramine treatment groups were gastrointestinal disorders and psychiatric disorders of mild to moderate severity. On-demand regimen of clomipramine 15 mg resulted in a significant improvement in IELT and was superior to a regimen of clomipramine 30 mg in terms of risk-to-benefit ratio.