A 2×2 factorial experiment with two ambient temperature levels (23 and 33 ℃) and two humidity levels (relative humidity were 55% and 80%) was conducted to study the effects of high ambient temperature and humidity on the growth performance, plasma cortisol concentration and immune function in growing pigs. Twenty-four cross-bred (Duroc×Landrace×Yorkshire) barrows with average body weight of (30.00±1.72) kg were randomly assigned to 4 treatments with 6 replicates each and 1 pig in each replicate. The experiment lasted for 14 days. Results showed as follows: 1) ambient temperature significantly affected the average daily feed intake (ADFI), average daily gain (ADG) and feed/gain (F/G) (P<0.01), and humidity significantly affected the ADG and F/G (P<0.01). Growth performance was decreased under high ambient temperature or high relative humidity, but there was no significant interaction between temperature and humidity (P>0.05). 2) Plasma cortisol concentration was significantly affected by temperature at the 1st day (P<0.01) and the 7th day (P<0.05). High ambient temperature increased the cortisol concentration at the 1st day, but decreased it at the 7th day. Temperature significantly influenced total white blood cells (TWBC) at the 7th day （P<0.05）, neutrophil and neutrophil to lymphocyte (N/L) ratio at the 1st day （P<0.01）, B lymphocyte transformation rate at the 1st day and the 7th day （P<0.05）, CD4+ cells and CD4+/CD8+ ratio （P<0.05） at the 14th day. Due to the high ambient temperature, TWBC was decreased at the 7th day, neutrophil and N/L ratio were increased at the 1st day, B lymphocyte transformation rate was increased at the 1st day and the 7th day, CD4+ cells and CD4+/CD8+ ratio were decreased at the 14th day. There were no significant effects under different humidity, and there was no significant interaction between temperature and humidity on plasma cortisol and immune function (P>0.05). It is concluded that the growth performance of growing pigs is decreased under high ambient temperature or high relative humidity. Furthermore, the constant high ambient temperature decreases cellular immune function, and thereby increases disease susceptibility.［Chinese Journal of Animal Nutrition, 2010，22（5）:1214-1219］