The current work was aimed to elucidate the structure of a yellow pigment LPT-3d from Lachnum sp. fermentation broth and antioxidant activities were discussed using H2O2-induce HepG2 cells. Based on UV-vis, FT-IR, HR-ESI-MS, NMR analysis, the yellow pigment was identified as a tetracyclic triterpenoid: methyl 2-(3-ethyl-4methyl-4-nitroimidazolidin-l-yl)-6,6,9,9a,11a-pentamethyl-1-(3-methyl-4,5-dihydro-3H-pyrazol-4-yl)-7,10-dioxo-8-(propionyloxy)hexadecahydronaphtho [1',2':6,7] indeno [1,7a-b]oxirene-5-carboxylate. The scavenging ability of DPPH was 95.65 +/- 0.51% (1000 mu g mL(-1)), and the I50 value of reducing power was 74.01 +/- 0.34 mu g mL(-1). HepG2 cell injury induced by H2O2 (300 mu M) for 2 h, the cell viability was 49.50% examined by MTT assay. The cell viabilities of LPT-3d treated groups (at the concentrations of 10, 20, 50 mu g mL(-1)) were 64.89%, 66.57% and 81.71% respectively. LPT-3d treated HepG2 cell at the contention of 50 mu g mL(-1) showed the activities of SOD and CAT were 39.98 +/- 3.02 U mg(-1) , 9.94 +/- 1.36 U mg(-1), and the level of MDA was 2.12 +/- 0.06 mg mg(-1) of protein. Compared with the control group, LPT-3d presented higher antioxidant activity and protective effect on H2O2-induced injury to HepG2 cells. The results suggested that Lachnum sp. triterpenoid yellow pigment had a potential application for the treatment of oxidative stress related hepatic diseases.