Abstract: Fat embolism syndrome （FES） is a severe complication after trauma and fracture. It is difficult to treat and capable of a high morbidity rate. If it is diagnosed and prevented or treated in the early stage, the pathological process may be stopped. The authors introduced a method by using rapid frozen section of venous blood clot stained with oil red “O”, then observed under light microscope. A positive result was accepted if some big or small orange fat droplets could be fond. With this method, 50 patients with traumatic fracture and 8 patients after total hip replacement were obsered. Fat droplets with diameter of 2-50 μm were found in 30 patients. 9 patients had the fat droplets in diameter of 20-50 μm and 6 of them suffered from FES. The other 3 patients were free of FES because dexamethasone or dextran were used in time. FES did not happen in another 21 patients whose fat droplets were positive but the diameter below 20 μm and in 28 fat droplets negative patients. So, a simple and practical method for early diagnosis of FES is recommended.