The purpose of this research was to study the effect of one session of a progressive endurance running in two environments (natural temperature and moderate temperature) on some markers of the clotting system (fibrinogen, activated thromboplastin time or APTT, prothrombin time or PT) in active girls. 27 physical education female students from Mazandaran University were selected and divided randomly into three groups: exercise in an environment with natural temperature or NTG group (23+2c), exercise in an environment with moderate temperature or HTG group (33+2c) and a group without exercises called HG which was only exposed to an environment with moderate temperature. Laboratory environment humidity was maintained in the range of 55+5% for all groups. The protocol was performed based on Karvonen formula with intensity 65%-75% VO2max for each subject on a non-gradient treadmill. The blood samples following 12 to 14 hours of a nightly fasting were gathered in a similar condition in three phases: the baseline level, mid-test and 30 minutes after the exercise. Coagulant manner was used to determine the levels of fibrinogen, APTT, and PT. The data were analyzed by repeated measures, ANOVA and t – student at P?0.05. The results showed that fibrinogen levels in three groups increased insignificantly in all phases. On the other hand, the increase of PT in NTG group in the mid-test and posttest and in NTG and HTG groups in posttest was significant when compared with the baseline level. Inter-group changes showed a significant difference in PT level between HG and HTG groups in the posttest phase. Based on these findings, it can be said that physical activity with moderate intensity induced the changes in clotting system, but mild heat stress did not affect the blood clotting system.