Yunnan Baiyao is a well-known Chinese herbal medicine that has been used as a haemostatic drug for nearly 100 years. The aim of this clinical trial was to evaluate the efficacy and safety of Yunan Baiyao capsules on the reduction of blood loss in bimaxillary orthognathic surgery. 87 consecutive patients scheduled for simultaneous maxillary Le Fort I osteotomies and bilateral sagittal split ramus osteotomies (BSSRO) were enrolled in a prospective, randomized, double-blind, placebo-controlled clinical trial. Patients were administered Yunnan Baiyao capsules or placebo capsules, orally for 3 days before surgery. Intraoperative blood loss was estimated and the safety of Yunnan Baiyao capsules was evaluated. The total blood loss in the Yunnan Baiyao group (mean, 330.5+/-134.4 ml) was significantly lower than in the control group (mean, 420.3+/-175.9 ml). No allergic reactions, thromboembolic events or other side effects were recorded in this trial. It can be concluded that the preoperative use of Yunnan Baiyao capsules, in combination with hypotension anaesthesia, results in a reduction in intraoperative blood loss in bimaxillary orthognathic surgery. Yunnan Baiyao capsules are an effective and safe haemostatic Chinese medicine.