The demand for Jersey cattle has increased among dairy farms in Republic of Korea because Jersey cattle are known to consume less feed and produce milk with better fat and protein ratio than Holstein Friesian cattle. Because the dairy industry using Jersey cattle is at the beginning stage, little information about Jersey cattle has been investigated in Republic of Korea. The aim of this study was to compare the differences of hematological values by breed, sex, and age. A total of 62 calves (27 female Holstein, 25 male Holstein, 4 female Jersey, 6 male Jersey) were used when they were 1 d and 1 mo old. All blood samples were collected through jugular vein at the NIAS in Cheonan, Korea. Procyte Dx analyzer and IBM SPSS 21.0 softeware were used. At d 1, female Holstein calves had an average of 7.8 RBC (M/µL) with 42.4 MCV (fL) and 9.0 WBC (K/µL) with 68.1% neutrophil (NE), 29.4% lymphocyte (LY) whereas male Holstein calves had 6.9 RBC with 42.1 MCV and 10.2 WBC with 67.5% NE, 28.1% LY. After a month, hematological values of female Holstein calves changed to 7.3 RBC with 33.6 MCV and 9.7 WBC with 35.2% NE, 54.3% LY while those of male Holstein calves turned to 7.0 RBC with 33.0 MCV and 9.7 WBC with 37.4% NE, 51.0% LY. As for Jersey, erythrocyte and leukocyte of female calves had changed from 7.4 RBC with 38.8 MCV and 7.5 WBC with 48.3% NE, 49.6% LY to 7.9 RBC with 34.8 MCV and 9.5 WBC with 25.8% NE, 58.0% LY for a month since a day old. Male Jersey calves had 6.8 RBC with 39.6 MCV and 9.7 WBC with 63.1% NE, 35.1% LY at d 1. After a month, male Jersey had 6.1 RBC with 30.8 MCV and 9.3 WBC with 29.1% NE, 55.7% LY. Female calves likely had more population of erythrocyte than male calves in both breeds. Holstein calves had bigger erythrocytes than Jersey calves at d 1. But, there was no big difference between male and female calves. The volume of erythrocytes had been reduced from d 1 to 1 mo regardless of sex and breed. The percentage of neutrophil had decreased whereas that of lymphocyte had increased for a month. The number of Jersey calves was so small that those results might not be convincing. Further works are required to study hematological and immunological differences between Holstein and Jersey.