In this study the effects of ovsynch protocol started in different phases of oestrus cycle on oestrus synchronization in Brown Swiss cows were examined. A total of 41 cows were randomly allocated into four groups. According to observation of the oestrus and ovulation, ovsynch protocol was began during oestrus (Group I, n=12), metoestrus (Group II, n=9), dioestrus (Group III, n=10) or proestrus (Group IV, n=10). All cows received 10 µg buserelin acetate intramuscularly and, 500 ng cloprostenol 7 d and a second GnRH 9 d after the first GnRH injection and AI 16 h after the second GnRH application. Ovarian structures were monitored daily during the experiment and pregnancies were diagnosed by ultrasound 28 d after AI. Ovulation or luteinization and new follicle development rates (%) after first GnRH injection are 100%, 78%, 100% and 90% in groups I, II, III and IV, respectively. On PGF2α injection day, 83.3%, 88.9%, 100% and 100% cows had large follicle (>8 mm) and 91.7%, 100%, 90% and 100% cows had lutein tissue in groups I, II, III and IV, respectively. Luteolytic responses to PGF2α, were 81.8%, 77.8%, 66.7% and 80% and ovulation responses to second GnRH were 83.3%, 66.7%, 80% and 70% in groups I, II, III and IV, respectively. Pregnancy rates were 58.3%, 44.4%, 40% and 50% in groups I, II, III and IV, respectively. There was only a significant difference for average diameter of corpus luteum on PGF2a injection day between groups II and IV (p<0.05). It is concluded that, ovsynch protocol could be began regardless of the phase of oestrus cycle.