A study was undertaken to determine the thermal inactivation of Escherichia coli O157:H7 as influenced by the effects of temperature, time, suspension medium and ascorbate. Tryptic soy broth was more heat resistant than pfosphate buffer (pH 7.1), with D values of 1.52~1.68 min at 6$0^{circ}C$ and 1.51~1.63 min at 7$0^{circ}C$ compared with 1.52~1.65 min at 6$0^{circ}C$ and 1.26~1.61 min at 7$0^{circ}C$ for phosphate buffer as suspension medium. E. coli O157:H7 was completely inhibited within 30 min when small inoculum (106 CFU/$mell$) was heated at 7$0^{circ}C$. When E. coli O157:H7 was preheated at 48$^{circ}C$ for 60 min in phosphate buffer before heating, D values were 1.28~1.60 min at 6$0^{circ}C$, and 1.13~1.56 min at 7$0^{circ}C$, 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 (109 CFU/$mell$), while ascorbic acid was no effect at low cell concentrations (109 CFU/$mell$).