The purpose of this study was to investigate the effects of environmental temperature and humidity on dairy cow''s rectal temperature, respiratory rate, and behavioral patterns. 20 lactating Holstein cows (160 ± 63 days postpartum) were used in the trial. In order to distinguish the heat stress level of cows, the temperature-humidity index, which can be measured by the temperature and humidity of the barn, was divided into three stages. Each stage was classified into 70~75 for the first stage (T1), 76~81 for the second stage (T2), and 82~87 for the third stage (T3). As a result, the average breathing rate per minute (times / min) was 58.60 in T1, 66.12 in T2 and 84.05 in T3, which increased as the temperature-humidity index increased. There was a significant difference between the test groups. The rectal temperature (℃) of the cows was 38.41 in T1, 38.69 in T2 and 39.05 in T3 (P < 0.05), indicating a positive correlation with the temperature-humidity index. The body surface temperature (℃) measured at the left cheek of the cow was 36.41 in T1, 36.51 in T2 and 37.39 in T3. There were no significant difference between T1 and T2, but T3 showed difference. The rumination time (min / day) was 473.10 in T1, 454.76 in T2, and 399.60 in T3 (P < 0.05). Daily lying time (min / day) was 658.76 in T1, 650.29 in T2 and 609.65 in T3. As the result shows, average lying time decreased as temperature-humidity index increased, but there was no significant difference among them. The mean standing time (min / day) was 781.24, 789.71, 830.35 in T1, T2, and T3, respectively. The milk yield per day (kg / d) was 33.52, 36.40, 30.19 in T1, T2, and T3, respectively. The milk yield of T1 and T3 showed significant difference. During past years, main research on dairy cows have been carried out to increase milk production. However, as the effects of climate changes are increasing recently, the dairy industry is expected to have continuous damage. Therefore, further research is needed to improve the welfare of dairy cows and reduce heat stress.