The temperature difference between the stem, leaf, fruit of tomato and the air at the same height was measured using the thermocouple temperature instrument, and the heat time constant of the tomato leaf was calculated. The result shows that the difference was clear, which was different at different parts of the plant. Fruit had the biggest temperature difference with the air at the same height whereas stem and leaf had the lower difference. The heat time constant of the tomato leaf was 172 s, whereas the absolute deflection is 73.1 s. The heat time constant is valuable because it is easier to be measured than the heat resistance and the general heat resistance can be obtained through its measurement.