Serum progesterone may increase prior to ovulation trigger in in vitro fertilization patients, jeopardizing endometrial receptivity and therefore live birth rate. Recombinant FSH (rFSH) promotes progesterone production from human granulosa cells. Yet, the role of FSH on progesterone production need deeper exploration. Studies were conducted in human primary cumulus cells from IVF cycles, human granulosa cell line and mice primary granulosa cells. The relative expression of let-7 was evaluated using real time PCR. Human primary cumulus cells were collected from individual cumulus-oocyte complex of high-progesterone patients (serum progesterone level higher than 5 nM, n=18) and control group (serum progesterone level less than 5 nM, n=25). The expression of let-7a in human primary cumulus cells was markedly reduced in the high-progesterone group compared to the control. The serum progesterone level was augmented after rFSH treatment at dose of 0.5, 1 and 2.5 IU along with reduce expression of let-7a. Progesterone level in cultured medium from isolated mouse primary granulosa cells and human granulosa cell line were significantly elevated with rFSH at 12.5, 25, 50 IU/L concentrations with decreased expression of let-7a. Besides, there was a robustly increase of let-7a expression in let-7a mimics-transfected group and decrease in let-7a inhibitor-group with or without rFSH treatment and the opposite trend of progesterone. Collectively, our findings revealed the key role of let-7 in rFSH induced progesterone level both in human and mouse granulosa cells, providing potential mechanism for premature progesterone rise.