Abstract: It is difficult to control the environment of boar houses in summer, because the houses are semi-enclosed with outside playing ground. In order to investigate the effect of air cooling fan on the reduction of heat stress in boar house, the field experiment was implemented in a typical boar house in Hebei Province. The results show that the cooling fan system can reduce the inside air temperature by 3～7℃ in the house without any construction change, and the temperature inside boar house is about 30℃ when the outside air temperature is above 35℃. The average effective temperature in boar house was 3℃ lower than outside effective temperature even in the extremely unfavorable weather with 90% of relative humidity.