Abstract: Eighty 25dayold male AA broilers were divided randomly into two groups (40×2): the control group (21～24℃, 50%±5% relative humidity) and thermostress group ((37±0.5) ℃,70%±5%relative humidity). The blood gas analysis, respiration rate, rectal temperature and production performance of broiler chickens were examined continuously.In thermostress group, 24 hours treatment induced a compensated respiratory alkalosis， which was characterized by the decreased partial pressure of CO2 （PaCO2）and the increased pH in artery; about 6 days later there was a decompensated metabolic acidosis, which was characterized by the dropped arterial blood bicarbonate ion (HCO-3) and the decreased pH value (P＜0.05); 12 days after thermostress acidbase balance was almost resumed. Body weight gain and feed intake of thermostressed chickens were decreased significantly (P＜ 0.01), and feed/gain was increased significantly (P＜0.01). The respiration rate and rectal temperature of heat-stressed broiler chickens were increased significantly (P＜0.01) at 1, 5, 7, 9 and 12 days after thermostress. The oxygen partial pressure （PaO2） had a significantly negative correlation with respiration rate (P＜0.01) and rectal temperature (P＜0.05). The arterial blood oxygen saturation（SaO2）had a significantly negative correlation with rectal temperature (P＜0.01). These results showed that there are the significant acidbase balance disturbance, the severely affected physiological function and decreased production performance in the broiler chickens under 37℃ constant heat-stress.