Background & Aim: Both exercise and unusual environments cause changes in the immune system function. The aim of this study was to compare the effect of exercise on the total numbers of blood Leukocytes and platelets of the athletes in cold, warm and normal temperature conditions. Methods: In this clinical trial, ten young male endurance athletes conducted the same exercise (treadmill running) for an hour at the intensity of %60 VO2 max in three normal, cold and warm temperature conditions. The number of Leukocytes, neutrophils, lymphocytes, monocytes and platelets were counted before, immediately and two hours after exercise (during recovery). The collected data were analyzed using the repeated measure and post hoc Bonferroni tests. Results: The total number of leukocytes increased significantly after exercise, in all circumstances (p0.05). During the rest period (recovery), the number of monocytes and lymphocytes decreased significantly in cold, normal and warm environments (p< 0.05) while decreases in the number of leukocyte, exception with neutrophils, was not significant in the warm environment (p<0.05). Conclusion: Exercise in cold, normal and warm environments caused stimulation and aggregation of immune cells. However, the exercise in warm environment increased the number of blood immune cells and also delayed the immune system in reaching the initial condition during the rest period, after the exercise.