Abstract: The 80% minimum inhibitory concentration of 22 5-(substitutedbenzyl)-2, 4-diamino-pyrimidines (16 of them are not reported in literatures) were tested using Escherichia coli bacteria (1515). The quantitative structure-activity relationships analysis shows that the sum of the steric parameter (ΣMR) of 3,4,5-substituents on phenyl group of benzylpyrimidines are bilinearly correlated with the inhibitory activity (n=20, r=0.84, s=0.345). The hydrophobicparameter(π)of the substituent cannot improve the correlation and the electronic parameter σR- of substituent gives slight contribution to the correlation.