Abstract: The objective of this study was to compare two strategies for resynchronization of ovulation based on ovsynch 48 and 56 h in non-pregnant dairy cow diagnoses using trans rectal ultrasonography Lactating Holstein cows (n = 150) were submitted for postpartum Timed Artificial Insemination (TAI) using a Ovsynch 48 and 56 h protocol. After diagnosed non-pregnant, cows were randomly assigned to initiate resynchronization. Non-pregnant cows as determined by ultrasonography on day 30 post-AI and were divided into 3 groups. Cows in 1st group receiving an injection of PGF2α in 30 days and detected in estrus were inseminated and 2nd group receiving an injection of PGF2α 7 days after the initial GnRH injection and a 2nd GnRH injection 48 h after the PGF2α injection and 3rd group receiving an injection of PGF2α 7 days after the initial GnRH injection and a 2nd GnRH injection 56 h after the PGF2α injection and cows in both treatments were inseminated approximately 16-20 h after the 2nd GnRH injection. Therefore, pregnancy rates were evaluated for 3 groups that 1st group (n = 50) 18 pregnant and 32 non-pregnant, pregnancy rate in this group 36% and cows in the group 2 (n = 50) 11 pregnant and 39 non-pregnant, pregnancy rate in this group 22% and cows in the group 3 (n = 50) 16 pregnant and 34 non-pregnant, pregnancy rate is 32% and pregnancy rate between group 1 and 2 was significant p = 0.0044 (p<0.05), pregnancy rate between group 2 and 3 was significant too p = 0.0446 (p<0.05) and pregnancy rate between group 1 and 3 was not significant p = 0.5202 (p>0.05).