The influence of insect-proof screens in the vents of a greenhouse on the microclimate and on human thermal comfort was investigated in this research. Komatsuna (Brassica campestris L.) was cultivated in the screened greenhouse and in an unscreened greenhouse. In the screened greenhouse, wind speed seldom exceeded 0.5m･s^-1 at a height of 1.5m, and the air temperature and vapor pressure deficits at a height of 0.2m were comparable to those in the unscreened greenhouse. However, the air-temperature difference between heights of 0.2 and 3.0m was much larger (8℃) in the screened greenhouse than in the unscreened greenhouse (2℃) during days with high solar radiation and low wind speed. The inside-outside temperature difference was much higher than that in the unscreened greenhouse when the outside wind speed was lower than 2 m･s^-1. The wet-bulb globe temperature (WBGT) in the screened greenhouse was 3 to 7 ℃ higher　than that　in　the unscreened　greenhouse　during　the　day. The level of human thermal comfort measured by WBGT in the screened greenhouse was improved by means of forced airflow directed onto the worker to promote convection, and by means of the addition of skylights to promote chimney-effect ventilation.