Anti-complementary activity-guided fractionation of the ethanolic extract of Prunella vulgaris spikes led to the isolation of a new, unusual delta11(12) triterpene, 3beta,13beta-dihydroxyolic-11-ene-28-oic acid (1), along with thirteen known triterpenes (2-14). The structure of the new compound was established by 1D and 2D NMR spectroscopic analysis. All the isolates were evaluated for their anti-complementary activity against the classical pathway (CP) and alternative pathway (AP). Eight triterpenes (1-8) showed anti-complementary activity against CP and AP, with CH50 and AP50 values of 0.15-0.37 mg/mL, and 0.29-0.53 mg/mL, respectively. Mechanism study using complement-depleted sera showed that oleanolic acid (2) acted selectively on C1q, C5, and C9 components, while 2alpha-hydroxy oleanolic acid (3) interacted with C1q, C3, C5 and C9, and 2alpha,3alpha-dihydroxyolic-12-ene-28-oic acid (4) blocked C1q, C2, C3, C5 and C9.