OBJECTIVE To separate and identify the chemical constituents of the n-butanol fraction from the stems of Clerodendrum trichotomum ，and to investigate their antitumor activities in vitro . METHODS The ethanol extracts were obtained with 85% ethanol from dried stems of C. trichotomum. After dispersed with water ，ethanol extracts were distributed by petroleum ether，ethyl acetate and n-butanol in turn ，then concentrated under reduced pressure to obtain the fractions of each extraction part . The n-butanol fraction from the stems of C. trichotomum was isolated and purified by macroporous resin D 101 column chromatography and various chromatographic techniques including silica gel ，hydroxypropyl glucan gel and Toyopearl HW -40F macroporous resin and so on . The structures of them were identified by physical and chemical properties ，MS and NMR . All these compounds were evaluated for cytotoxic activities against 4 kinds of human tumor cells such as cultured K 562，MCF-7，A549 and HepG2，using the MTT assay . RESULTS Fourteen chemical constituents were isolated and identified as teuvincenone B （1）， uncinatone（2），villosin C （3），syringaresinol（4），syringaresinol-4ʹ-O-β-glucopyranoside（5），3，12-O-β-D-diglucopyranosyl-11，16- dihydroxyabieta-8，11，13-triene（6），glypentoside C （7），martynoside（8），isomartynoside（9），2-（4-hydroxyphenyl）ethanol-O-β-D- glucopyranosyl-（1→2）-O- β -D-glucopyranoside（10），3，4-dimethoxyphenyl-1-O- β -D-apiofuranosyl （1→2） - β -D- glucopyranoside（11）， 2，6-dimethoxy-4-hydroxy-1-O- β -D- glucopyranoside（12），adenosine（13）and cistanoside F （14）. In vitro anti-tumor activity studies showed that compounds 1-3 showed certa in inhibitory activities against tumor cellproliferation，among which compound 2 displayed the strongest inhibitory activity against MCF -7，A549 and HepG 2 cells，and their IC 50 values were 25.00，22.34 and 12.50 μmol/L respectively ；only compound 3 showed stronger inhibitory activity against K562 cell with IC 50 of 28.41 μmol/L. CONCLUSIONS Among them ，compounds 10 to 13 are isolated from genus Clerodendrum for the first time ，compounds 4，5，14 were isolated from C. trichotomum for the first time . The abietane diterpenoids （compounds 1-3）have better inhibitory activities against above four tumor cell lines .