Abstract: The effects of air relative humidity（RH）on the photosynthesis and fruit-set of tomato seedling under high temperature were investigated in growth chambers. Three air relative humidity treatments from 10:00 to 16:00 were 70% ± 5%（high RH），55% ± 5%（middle RH）and non-humidification
（low RH，as control）. The results showed under the high temperature（the average temperature from 10:00 to 16:00 was 33–43 ℃），high RH treatment significantly promoted transpiration rate and net photosynthetic rate，also decreased or eliminated the photosynthetic midday depression compared with low RH treatment.Hence，the cumulative value of transpiration and photosynthesis from 9:00 to 17:00 of high RH treatment were 116%–133% and 31%–343% higher than that of low RH treatment，respectively. The
fruit-set rate of high RH treatment was up to 48% and significantly higher than low RH treatment. However, photosynthesis and fruit-set rate of middle RH treatment were not significantly different compared with that of low RH treatment. As a conclusion，relative humidity of 70%（high RH）improved
photosynthesis and fruit-set of tomato under high temperature above 30 ℃.

Key words: tomato, high temperature, air relative humidity, photosynthesis characteristics, fruit-set