Abstract: To know the cause of the plant temperature variation, the daily changes of the heat conducted within the tomato fruit and stem in the greenhouse were measured using the thermocouple and Li-cor's automatic meteorology station. The temperature differences between the stems and the air were measured, and the convection heat exchanges from the plant to air were calculated according to the theory of biological heat conduction. The results were that there was temperature difference between the surface and the center of the tomato fruit, which had daily changes, and the biggest heat conduction of tomato fruit was 1.29 W, which was about the 0.2% of the solar radiation in the greenhouse during the noon in a clear day. The heat convections between the tomato plant and the environment in different weather conditions had the same trend. In clear day, during the night, the heat convection was at steady loss, at the time of 10∶30, it lost most, -18.9 W/m²; at 11∶15 it started to get heat, at 13∶30 it was at the peak of getting heat 34.9 W/m²; from 18∶30 to the next morning, it lost heat. In the overcast day, the biggest and smallest values were reduced. In the cloudy day, the result was within the clear day and the overcast day, and it contained clear fluctuation. The heat convection was bigger than the heat conduction.