The aim of this study is to investigate whether a natural cycle intrauterine insemination (NC-IUI) monitored with ultrasonography and serum estradiol level can improve the pregnancy rate (PR) of NC-IUI. A total of 311 cycles of 135 cases was treated with a NC-IUI using a ultrasonographic monitor for follicular development (Group A). And a total of 308 cycles of 131 cases underwent a NC-IUI monitored baseline ovarian cysts, follicular development and ovulation using ultrasonography and serum estradiol level (Group B). Forty-two cycles (12.0%) were cancelled due to baseline ovarian cysts and 20 cycles (6.1%) were excluded from group B because of a luteinized unruptured follicle. In group A and B, 16 pregnancies (5.1% per cycle) and 32 pregnancies (10.4% per cycle) were obtained respectively. The differences between the PRs of the two groups attained statistical significance (P < 0.05). Our data indicated that a NC-IUI monitored with ultrasonography and serum estradiol level may improve the PR of NC-IUI.