In order to determine air temperature difference by canopy layer in the forest, air temperatures were observed at Seolleung Park, Gahngnam-ku, Seoul. from November 9, 2007 to November 8, 2008 by 10 minute interval. The data were analyzed in terms of diurnal variation based on annual and monthly temperature difference. Using calm, less cloudy and no rainy weather data, average air temperature difference between forest and grass was observed as $0.8^{circ}C$. The maximum air temperature difference was observed at 22:10, 23:20, 23:30 and 23:40 by $2.13^{circ}C$ and the minimum one observed at 13:00 by $-0.84^{circ}C$ in diurnal variation. The maximum temperature difference occurred at 19 : 50 on September by $3.67^{circ}C$, Overall the air temperature in the forest was higher than that of grass at night and lower in midday.