The 70% ethanol extract from Adenophora triphylla showed a strong antioxidant effect and reduced cytotoxicity of H2O2 in SK-N-SH cells. The chloroform fraction from A. triphylla extract (AT-CH) among the six fractions showed strong DPPH radical and intracellular reactive oxygen species (ROS) scavenger effects and the highest protective effect against H2O2-induced SK-N-SH cell death. Bioactivity compounds were purified from AT-CH, and the chemical structures of the compounds were determined as β-sitosterol and daucosterol on the basis of 1H-NMR, 13C-NMR, and EI mass spectra. β-Sitosterol and daucosterol also had protective effects against oxidative stress in SK-N-SH cells. Phospho-p38 MAPK levels were elevated by H2O2 but were inhibited by treatment with AT-CH and phytosterols (β-sitosterol and daucosterol) isolated from AT-CH. These results suggest that AT-CH has brain neuroprotective effects against oxidative stress (H2O2) by inhibiting activation of p38 pathways and scavenging intracellular ROS.