Two active substances from ginseng fermented using Ganoderma lucidum mycelia were investigated for antiproliferative effects against the human lung cancer cell line A549. The chloroform fraction of fermented ginseng extracts showed a strong antiproliferative effect. This fraction was isolated and purified using silica gel and C18 resin column chromatography and semi-preparative reverse phase HPLC. The structures of isolated compounds were determined using spectroscopic methods (ESI-MS, 1 H and 13 C NMR). Isolated compounds were identified as ginsenoside compound K and 3-oxo-compound K. Both inhibited A549 cell growth in a dose-dependent manner. Cell viability values for ginsenoside compound K were 74.88, 59.30, 5.76, 5.79, and 6.27% at 6.25, 12.50, 25.00, 50.00, and 100.00 μg/mL, respectively, and ginsenoside 3-oxo-compound K showed values 89.40, 59.62, 6.05, and 4.64% at 3.70, 7.50, 15.00, and 30.00 μg/mL, respectively. Compound K and 3-oxo-compound K from fermented ginseng can be used as natural anti-cancer agents.