Hypothermic circulatory arrest (HCA) is an essential procedure during aortic surgery to protect organs; however, hypothermia is believed to cause coagulopathy, which is a major fatal complication. This study aimed to clarify the impact of hypothermia on coagulation by eliminating clinical biases in vitro. In the hypothermic storage study, blood samples from five healthy volunteers were stored at 37 ℃ (group N) for 3 h or at 20 ℃ for 2 h, followed by 1 h of rewarming at 37 ℃ (group H). Thromboelastography was performed before and after 3 h of storage. In the mock circulation loop (MCL) study, blood samples were placed in the MCL and (a) maintained at 37 ℃ for 4 h (group N, n = 5), or (b) cooled to 20 ℃ to simulate HCA with a 0.1 L/min flow rate for 3 h and then rewarmed to 37 ℃ (group H, n = 5). The total MCL duration was 4 h, and the flow rate was maintained at 1 L/min, except during HCA. Blood samples collected 15 min after the beginning and end of MCL were subjected to standard laboratory tests and rotational thromboelastometry analyses. Hypothermia had no impact on coagulation in both the hypothermic storage and MCL studies. MCL significantly decreased the platelet counts and clot elasticity in the INTEM and EXTEM assays; however, there was no effect on fibrinogen contribution measured by FIBTEM. Hypothermia does not cause irreversible coagulopathy in vitro; however, MCL decreases coagulation due to the deterioration of platelets.