At present, Vietnam's dairy industry is developing but faces with several reproduction disorder problems including persistent corpus luteum, cystic ovary or inactive ovary. An experiment was carried out ffom Apr. 2011 to Apr. 2014 to study the effects of season, calving parity and body score on ovary activities of 120-day post-calving dairy cows reared at Bavi, Hanoi. Clinical examination of 746 cows showed that, 16.75 percent of them had no estrous signs until 120-day post-calving owing to ovarian problems. The percentages of ovarian disorder in Spring and Summer were higher than that in Autumn and Winter and also increased following the calving parities. In addition, the pathological symptoms of ovaries of the cows were affected by the body score, i.e., too fat or too thin cows.