A new lanostan-type triterpenoid with hydroperoxy group, toosendanin A (1), together with two known triterpenoids, meliastatin 3 (2) and ursolic acid (3), were isolated and identified from the stems of Melia toosendan. The structures of these compounds were elucidated by 1D- and 2D-NMR spectra and other spectroscopic studies. These compounds were assayed for the antibacterial activities against some hospital pathogenic bacteria. Toosendanin A (1) exhibited strong antibacterial activity against K. pneumoniae.