The experiment was conducted to clarify the diurnal variations of leaf temperatures of rice (C₃ plant) and Japanese barnyard millet (C₄ plant). Both plants are hygrophyte and grow under similer conditions. The rice and Japanese barnyard millet were grown in pots under flooded condition. One month after transplanting, the leaf temperatures and transpiration rates were measured under natural conditions. Solar radiation, wind speed, air temperature and relative humidity were also measured. The period of the measurements were from July 25 to July 31, 1977. The results obtained are as follows:<br>1) Diurnal variation of leaf temperatures of rice and Japanese barnyard millet;<br>The leaf temperature of rice was almost equal to the air temperature at sunrise and sunset and 0.5 to 1.0°C higher at night. However, it came below the air temperature with increasing solar radiation. The leaf temperature of Japanese barnyard millet was nearly the same as the air temperature at night. However, it went up higher than the air temperature after sunrise, and became increasingly higher with solar radiation.<br>2) Diurnal variation of transpiration rates of rice and Japanese barnyard millet;<br>There was no difference in transpiration rate between rice and Japanese barnyard millet under weak solar radiation. In the midday of high solar radiation, the transpiration rate of rice was greater than that of Japanese barnyard millet.<br>3) Relationship between solar radiation and leaf temperatures of rice and Japanese barnyard millet;<br>The temperature difference between leaf of rice and air was correlated negatively with solar radiation. The temperature difference between leaf of Japanese barnyard millet and air was correlated positively with solar radiation.<br>4) Relationship between solar radiation and transpiration rates of rice and Japanese barnyard millet;<br>The transpiration rates of rice and Japanese barnyard millet were correlated positively with solar radiation. Transpiration rate of rice was approximately the same as that of Japanese barnyard millet under low solar radiation, whereas it was higher under high solar radiation.<br>5) Relationship between leaf temperatures and transpiration rates;<br>The difference of leaf temperature between Japanese barnyard millet and rice was correlatedpositively with difference of transpiration rates between rice and Japanese barnyard millet.<br>Rice and Japanese barnyard millet are known to have similar geographical distributions, although the optimum temperatures for photosynthesis and growth of rice are lower than those of Japanese barnyard millet. They also share the same growing season. These facts could be, we suppose, due to the lower plant temperature of rice under high air temperature and solar radiation.