Antibacterial activity of 17 cinnamic acid related compounds against Bacillus subtilis and Escherichia coli was investigated to elucidate the structure responsible for the activity. Absolute hardness (η) was well correlated with minimum inhibitory concentration (MIC) expressed in logarithum. The correlation coefficient (r) between log (MIC) and η was 0.944 for B.subtilis and 0.932 for E.coli. Considering η and structure, the compound having the substituted phenyl group in propiolic aldehyde or its ester form should possess the strongest antibacterial activity.