The aim of this study was to determine short-term changes of blood rheology after intensive endurance exercise in national triathlon athletes. Nine national athletes of triathlon (mean age=20.12±2.15 years, weight 70.12±8.54 kg) took part in this study voluntarily. Subjects performed triathlon race which is one of the branches of multiple sports (which consisted of 2 kilometers running, 7 kilometers cycling and 2 kilometers continuous running). Blood samples (7 mm) were collected at the baseline (before) and after the race from antecubital vein. Blood viscosity, RBC, hematocrit, hemoglobin and ESR were measured. The data were analyzed by paired sample t test. The findings showed that after the race, blood viscosity significantly increased at 12, 30 and 60 speed (P<0.05). RBC (red blood cell), hemoglobin and HCT increased significantly after the race (P<0.05). Plasma volume and ESR (erythrocyte sedimentation rate) decreased significantly (P<0.05). It can be concluded that maximal endurance training specifically influenced hematological factors and increased viscosity and changes in blood components.