A study was undertaken to determine the thermal inactivation of Escherzchia coli O157:H7 as influenced by the effects of temperature, time, suspension medium and ascorbate. Tryptic soy broth was more heat resistant than phosphate buffer (pH 7.1), with D values of 1.52-1.68 min at 60℃ and 1.51-1.63 min at 70℃ compared with 1.52-1.65 min at 60℃ and 1.26-1.61 min at 70℃ for phosphate buffer as suspension medium. E. coli O157:H7 was completely inhibited within 30 min when small inoculum (10^6 CFU/ml) was heated at 70. When E. coli O157:H7 was preheated at 48℃ for 60 min in phosphate buffer before heating, D values were 1.28-1.60 min at 60℃, and 1.13-1.56 min at 70℃, showing that preheating increases the heat resistance of the strain. Phosphate buffer containing ascorbate (0.001 M) was enhanced the thermal inactivation of the strain when inoculated as large inoculum (10^9 CFU/ml), while ascorbic acid was no effect at low cell concentrations (10^6 CFU/ml).