The objective of this study was to investigate the resumption of postpartum ovarian cyclicity, and to determine the relationship between concentrations of plasma urea nitrogen (PUN) and resumption of ovarian cyclicity in Holstein cows. The cows were considered to have resumed ovarian cyclicity on the day of ovulation, if followed by regular ovarian cycles. 58.8 percentage of the cows (114/194) had normal resumption of ovarian cyclicity (resumption within 40days after calving), and 41.2% (80/194) had delayed resumption (resumption did not occur until> 40days after calving). Delayed resumption Type I (one or more ovarian cycles with luteal phase> 20days, i.e. prolonged luteal phase; 17.5%) and delayed resumption Type II (first ovulation did not occur until ${ge}40days$ after calving, i.e. delayed first ovulation 22.7%) were the most common types of delayed resumption. 18 percentage of the cows (35/194) did not resume their ovarian cyclicity until 60days postpartum. Prolonged luteal phase and delayed first ovulation were two important ovarian dysfunctions that delayed postpartum resumption of cyclicity in dairy cows. Cows with PUN of <15, $15{sim}19.9;and;{ge}20mg/dl$ had the likelihood ratios of normal ovarian cyclicity of 0.9, 1.14 and 0.55, respectively. Thus, PUN concentration of $15{sim}19.9mg/dl$ had a favorable association with postpartum resumption of cyclicity, whereas lower or higher PUN had a negative association with postpartum resumption of cyclicity.