To study the constituents in the chloroform extract of olibanum and their antitumor activities.The compounds were isolated by chromatographic methods and their structures were identified on the basis of spectroscopic methods and X-ray diffraction. The antiproliferative effect of the compounds in human leukemia HL-60 cells was tested by viable cell counting.Four cembrane diterpenes were isolated and identified as incensole-oxide (1), acetyl incensole-oxide (2), incensole (3), and acetyl incensole (4).Compounds 2 and 4 were isolated from the genus Boswellia for the first time. Compound 4 showed growth inhibitory effect against human leukemia HL-60 cell lines with IG50 value of (16.3 +/- 3.4) micromol x L(-1).