Abstract: The 95% ethanol extract of the dried fruiting bodies of Boletus bainiugan Dentinger was separated and purified by silica gel column chromatography, MCI column chromatography, open ODS column chromatography, Sephadex LH-20 column chromatography, preparative thin-layer chromatography and semi-preparative high-performance liquid chromatography. The structures of the compounds were identified by NMR and ESI-MS. 21 compounds were isolated and identified, including 9 ergosterols and 12 nitrogen-containing compounds, namely ergosterol peroxide (1), (24S)-ergost-7-en-3β-ol (2), (22E,24R)-ergosta-7,22-dien-3β-ol (3), (22E,24R)-ergosta-5,7,22 -triem-3β-ol (4), (3β,5α,6β,22E)-ergosta-7,22-diene-3,5,6-triol (5), 3β,5α-dihydroxy-(22E,24R) -ergosta-7,22-dien-6-one (6), (3β,5α,6β,22E)-6-methoxyergosta-7,22-diene-3,5-diol (7), volemolide (8), (17R)-4-hydroxy-17-methylincisterol (9), 5'-deoxy-5'-methylamino-9-(α-L- lyxofuranosyl) adenine (10), 5'-deoxy-5'-methylamino-adenosine (11), β-adenosine (12), uridine (13), 5'-S-methyl-5'-thioadenosine (14), 5'-O-acetyluridine (15), 2-formyl-1H-pyrrole- 1-butanoic acid (16), leptosphaerin (17), zarzissine (18), uracil (19), nicotinic acid (20) and 2-pyrrolidone (21). All of 21 compounds were isolated from this species of Boletus bainiugan Dentinger for the first time. The cytotoxic activities of compounds 1, 4-7, 9, 12-14, 19-21 against liver cancer cell line HepG2 and breast cancer cell line MDA-MB-231 were evaluated by MTS assay. The results showed that compounds 7 and 9 had half inhibitory activities against the in vitro growth of both tumor cell lines. The inhibition rates of compound 7 against HepG2 and MDA-MB-231 were (78.07 ± 0.71)% and (53.16 ± 0.76)%, respectively, with IC₅₀ values of (23.03 ± 0.41) μmol·L⁻¹ and 14.20 ± 0.16 μmol·L⁻¹, respectively. The inhibition rates of compound 9 against HepG2 and MDA-MB-231 were (96.50 ± 0.54)% and (72.94 ± 1.26)%, respectively, with IC₅₀ values of (29.86 ± 2.68) μmol·L⁻¹ and (12.11 ± 0.62) μmol·L⁻¹, respectively.