This study was carried out to investigate the effect. of L-cysteine and sodium sulfite on heat inactivation of soybean trypsin inhibitor(STI) and to determine cultivar difference in the inhibitory activity of STI. Effect of L-cysteine and sodium sulfite at different concentrations, pH`s, and lengths of treatment on inactivation of STI were studied. The inactivation of STI was spectrophotometrically determined by measuring the rate of production of p-nitroaniline from synthetic substrate, N-benzoyl-DL-arginine-p-nitroanilide. Addition of L-cysteine and sodium sulfite increased magnitude of heat inactivation and greatly inhibited the re-activation of STI. There was no difference STI inactivation in among soybean cultivars employed. The trypsin inhibitory activity of STI of the soybean cultivars ranged from 64.7 to 86.4 TIU(trypsin inhibitor unit) per gram soyflour and the decreasing order of the TIU was Jangback$gt;Hill$gt;Jangyeab, Kwangkyo$gt; Danyeab$gt;Dangkyung$gt;Paldal, Saeal, Duckyu$gt;Hwangkeum. Inhibitory activity of STI was correlated with cysteine content(r=0.6568^*) and with digestivility(r=-0.7695^(**)), but there was no correlation between the protein content and the inhibitory activity of STT.