To estimate the quality of the emergency medical services system of Wonju City, we studied the diurnal variations of 179 non-traumatic cardiac arrest victims who received cardiopulmonary resuscitation at the emergency center of Wonju Christian Hospital. Diurnal variations of non-traumatic cardiac arrest patients were as follows : The occurrence of cardiac arrest at day-time was higher than night-time; 18 cases (11%) from midnight to AM 4, 25 cases (14%) from AM 4 to AM 8, 42 cases (24%) from AM 8 to AM 12, 46 cases (25%) from AM 12 to PM 4, 35 cases (19%) from PM 4 to PM 8, 13 cases (7%) from PM 8 to midnight. Witness cardiac arrest was increased more during the day than night; 40% from midnight to AM 4, 48%from AM 4 to AM 8, 57% from AM 8 to AM 12, 52%from AM 12 to PM 4, 60% fmm PM 4 to PM 8, 38% from PM 8 to midnight. The transportation time at night-time cardiac arrest was more longer than day-time cardiac arrest; 3012mins from midnight to AM 4, 26+-9mins from AM 4 to AM 8, 27+-12mins AM 8 to AM 12, 25+-11mins from AM 12 to PM 4, 25+-9mins from PM 4 to PM 8, 3515mins from PM 8 to midnight. The rate of restoration of spontaneous circulation(ROSC) in day-time cardiac arrest was higher than the night-time cardiac arrest; 30% from midnight to AM 4, 36% from AM 4 to AM 8, 32%AM 8 to AM 12, 44% from AM 12 to PM 4, 41% from PM 4 to PM 8, 15% from PM 8 to midnight. The survival rate of cardiac arrest has been correlated with collapse time, early bystander CPR, earh advanced care. To improve outcome for prehospital cardiac arrest, we concluded that early bystander CPR, and early advanced life support should be performed at the scene and during the transportation especially at night.