The objective was to study the effect of ovulatory follicle diameter (OFD) at the time of timed-AI (TAI) on the fertility response in Ovsynch-TAI cows in comparison with spontaneous estrous/ovulation cows inseminated upon heat detection (SOHD-AI). Fifty Holstein cows were equally assigned to Ovsynch-TAI and control SOHD-AI groups (n=25 for each). Transrectal ultrasonography (TUS) was performed to estimate OFD on Day 0 and corpus luteum volume (CLV) on Day 13 and diagnose pregnancy on Day 28-32 post-insemination. Blood samples were obtained on Day 0 for estradiol (E2) assay and on Days 5 and 13 for progesterone (P4) assay. The serum progesterone concentration was greater in conceived versus non-conceived group on Days 5 (p = 0.002) and 13 (P = 0.001) in the Ovsynch-TAI group and on Day 13 (P = 0.002) in SOHD-AI group. There is a positive correlation between OFD and CLV (P =0.02) and between CLV and serum progesterone concentration (P = 0.01) simultaneously in the Ovsynch-TAI group. The conception rate was 32% and 24% in Ovsynch-TAI and SOHD-AI groups respectively. It is concluded that OFD at the time of TAI could be used to predict the fertility response to Ovsynch protocol since it is positively correlated to CLV on Day 13 which in turn is correlated to serum P4 concentration on Day 13 post -TAI.