It is believed that a lower temperature setting of hypothermic circulatory arrest (HCA) in thoracic aortic surgery causes coagulopathy, resulting in excessive bleeding. However, experimental studies that eliminate clinical factors are lacking. The objective of this study is to investigate the influence of the temperature setting of HCA on coagulation in a pig model. Ten pigs were divided into the following two groups: moderate temperature at 28 °C (group M, n = 5) or lower temperature at 20 °C (group L, n = 5). Two hours of HCA during a total of 4 h of cardiopulmonary bypass (CPB) were performed. Blood samples were obtained at the beginning (T1) and the end (T2) of the surgery, and coagulation capability was analyzed through standard laboratory tests (SLTs) and rotational thromboelastometry (ROTEM). In SLTs, hemoglobin, fibrinogen, platelet count, prothrombin time, and activated partial thromboplastin time were analyzed. In ROTEM analyses, clotting time and clot formation time of EXTEM, maximum clot firmness (MCF), and maximum clot elasticity (MCE) of EXTEM and FIBTEM were analyzed. Fibrinogen decreased significantly in both groups (group M, p = 0.008; group L, p = 0.0175) at T2, and FIBTEM MCF and MCE also decreased at T2. There were no differences regarding changes in parameters of SLTs and ROTEM between groups. CPB decreases coagulation capacity, contributed by fibrinogen. However, a lower temperature setting of HCA at 20 °C for 2 h did not significantly affect coagulopathy compared to that of HCA at 28 °C after re-warming to 37 °C.