The aim of this study was to assess the precision of the estimates of the time of estrous cycle, optimal breeding and ovulation derived by vaginal cytology. The thirteen Korea Jin-do dogs were examined the vaginal cytology, plasma estradiol-17$$eta $ and progesterone assay during the estrous cycle. Day 0 was the day of the first male acceptance. The main change of vaginal cytology during the estrous cycle was the high proportion of anuclear cell and erythrocyte in proestrus, superficial cell, anuclear cell and erythrocyte in estrus, parabasal cell, large intermediate cell and leukocytes in diestrus, and parabasal cell and small intermediate cell in anestrus, respectively. These data indicated that vaginal cytology was reliable method for estimating estrous cycle in Korea Jin-do dogs. In the cell indices during estrus the maximum eosinoghilic index was $92.0{pm}$2.6 (Mean{pm} SEM$)% at Day 2 and the maximum cornification indez was $96.0{pm}1.3%$ at Day 2, respectively. The eosinothilic indez and cornification indez of up to 70% were found at Day -1 to Day 5 and Day -6 to Day 8, and up to 80% at Day 1 to Day 4 and Day -4 to Day 6, respectively. From these data it was presumed that eosinophilic index was more reliable index for monitoring optimal breeding time than cornification indexm because eosinophilic index peak period was shorter than cornification indeX peak period and Day 2 was the day of ovulation. Therefore, optimal breeding time was the eosinophilic index peak period, more than 80% of eosinoghilic index. The $estradiol-17{eta}$ peak, with 3 days delayed when progesterone concentration was $4.5{pm}0.5 ng/ml$. These data estimated that the ovulation time was the day of eosinophilic index peak, Day 2. breeding time and pvulation time in Korea Jin-do dogs.