In this study, a comparison of characteristics of temporal changes in temperature, discomfort index (DI), and wet bulb globe temperature (WBGT) between Seoul and Daegu during the 2018 heat wave was conducted. Both Seoul and Daegu showed high temperatures exceeding the heat wave warning level (33°C) from 11:00 to 19:00. However, the time slot when the highest daily temperature was observed differed between the two regions; it was around 14:00 for Daegu and around 16:00 for Seoul. There was also a significant difference in the time slot when the WBGT value exceeded the severe warning level (28°C or higher) in the two regions; it was from 09:00 to 18:00 for Daegu and from 11:00 to 20:00 for Seoul. The time slot when the highest value was observed was 14:00 for Daegu and 15:00 for Seoul. These data indicate that the time slot when the thermal environment of the heatwave warning level was observed later in Seoul than in Daegu, but the duration in Seoul was longer than in Daegu. Likewise, the time-period when the highest level of the high-temperature thermal environment was observed was 2~3 h later in Seoul than in Daegu. The DI value at which more than 80% of the people felt discomfort due to high temperatures was observed from 09:00 to 22:00 in both regions, while the highest DI value at which all people felt discomfort due to high temperatures was observed only in Seoul from 14:00 to 18:00. The difference in the thermal environment between the two regions is believed to be mainly because there is a substantially higher artificial structure density in Seoul than in Daegu; therefore, the urban heat capacity is much higher in Seoul than in Daegu.