Abstract: As part of living organisms, animals require complex environmental conditions and they have evolved mechanisms to cope with stress caused by changing environmental conditions. Environmental stress includes but not limited to being separated from dams, adapting to novel surroundings, struggling for rank and housing conditions. These factors interacting with genetic background can reduce the growth performance, decrease immune response and elevate cortisol concentration of an animal leading to compromised productivity in mass. This study was carried out so as to investigate the effects of different seasons on the growth performance, blood characteristics and immune responses in fattening pigs. Crossbred barrows of approximately 85 kg body weight were divided into 4 groups for each season (6 pigsx3 pensx4 seasons) and the diet was given for 4 weeks. The blood samples were collected from 4 pigs in each pen 2 h after the last feeding. the temperature inside was 23.8, 35.7, 22.1 and 11.8°C in spring, summer, autumn and winter, respectively when blood samples were collected. Summer significantly lowered (p<0.05) Average Daily Gain (ADG) and feed efficiency as well as concentrations of platelets, Immunoglobulin M (IgM), Immunoglobulin G (IgG) and interferon-gamma (IFN-γ) compared with other seasons. Summer also significantly increased (p<0.05) concentrations of leukocytes, Lactate Dehydrogenase (LDH) and cortisol compared with other seasons. Therefore, these results indicated that the different seasons enhanced growth performance, blood characteristics and immune responses and summer was more stressful than spring, autumn and winter in fattening pigs.