Cancer is a major health obstacle around the world, with hepatocellular carcinoma (HCC) and colorectalcancer (CRC) as major causes of morbidity and mortality. Nowadays, there isgrowing interest in the therapeuticuse of natural products for HCC and CRC, owing to the anticancer activity of their bioactive constituents.Boswellia serrata oleo gum resin has long been used in Ayurvedic and traditional Chinese medicine to alleviatea variety of health problems such as inflammatory and arthritic diseases. The current study aimed to identifyand explore the in vitro anticancer effect of B. Serrata bioactive constituents on HepG2 and HCT 116 cell lines.Phytochemical analysis of volatile oils of B. Serrata oleo gum resin was carried out using gas chromatographymassspectrometry (GC/MS). Oleo-gum-resin of B. Serrata was then successively extracted with petroleumether (extract 1) and methanol (extract 2). Gas-liquid chromatography (GLC) analysis of the lipoidal matterwas also performed. In addition, a methanol extract of B. Serrata oleo gum resin was phytochemically studiedusing column chromatography (CC) and thin layer chromatography (TLC) to obtain four fractions (I, II, IIIand IV). Sephadex columns were used to isolate β-boswellic acid and identification of the pure compound wasdone using UV, mass spectra, 1H NMR and 13C NMR analysis. Total extracts, fractions and volatile oils of B.Serrata oleo-gum resin were subsequently applied to HCC cells (HepG2 cell line) and CRC cells (HCT 116 cellline) to assess their cytotoxic effects. GLC analysis of the lipoidal matter resulted in identification of tricosane(75.32%) as a major compound with the presence of cholesterol, stigmasterol and β-sitosterol. Twenty two fattyacids were identified of which saturated fatty acids represented 25.6% and unsaturated fatty acids 74.4% of thetotal saponifiable fraction. GC/MS analysis of three chromatographic fractions (I,II and III) of B. Serrata oleogum resin revealed the presence of pent-2-ene-1,4-dione, 2-methyl- levulinic acid methyl ester, 3,5- dimethyl- 1-hexane, methyl-1-methylpentadecanoate, 1,1- dimethoxy cyclohexane, 1-methoxy-4-(1-propenyl)benzene and17a-hydroxy-17a-cyano, preg-4-en-3-one. GC/MS analysis of volatile oils of B. Serrata oleo gum resin revealedthe presence of sabinene (19.11%), terpinen-4-ol (14.64%) and terpinyl acetate (13.01%) as major constituents.The anti-cancer effect of two extracts (1 and 2) and four fractions (I, II, III and IV) as well as volatile oils of B.Serrata oleo gum resin on HepG2 and HCT 116 cell lines was investigated using SRB assay. Regarding HepG2cell line, extracts 1 and 2 elicited the most pronounced cytotoxic activity with IC50 values equal 1.58 and 5.82μg/mL at 48 h, respectively which were comparable to doxorubicin with an IC50 equal 4.68 μg/mL at 48 h. Withrespect to HCT 116 cells, extracts 1 and 2 exhibited the most obvious cytotoxic effect; with IC50 values equal0.12 and 6.59 μg/mL at 48 h, respectively which were comparable to 5-fluorouracil with an IC50 equal 3.43 μg/mL at 48 h. In conclusion, total extracts, fractions and volatile oils of B. Serrata oleo gum resin proved theirusefulness as cytotoxic mediators against HepG2 and HCT 116 cell lines with different potentiality (extracts >fractions > volatile oil). In the two studied cell lines the cytotoxic acivity of each of extract 1 and 2 was comparableto doxorubicin and 5-fluorouracil, respectively. Extensive in vivo research is warranted to explore the precisemolecular mechanisms of these bioactive natural products in cytotoxicity against HCC and CRC cells.