In searching for anticomplementary compounds, three phenylpropanoids were isolated from the roots of Agastache rugosa and identified as rosmarinic acid (RA), rosmarinic acid methyl ester (RAM) and caffeic acid methyl ester (CAM) by NMR analyses. RA and RAM exhibited strong inhibitory activity on both the classical pathway (CP) and the alternative pathway (AP) of the complement system, in vitro, but CAM did far less than RA and RAM. $RAM-M1{sim}-M5$, the methylated derivatives from the RAM, showed that the inhibitory activity was decreased in inverse proportion to the number of methylated groups and $RAM-M 2{sim}-M4$, the isomers of two methylated hydroxyl groups, exhibited different inhibition activity.