The dried roots of Euphorbia kansui (kansui) have been used for centuries in China as a herbal medicine for edema, ascites, and asthma. The 95% ethanol extract showed a significant inhibition of cell proliferation against human normal cell lines L-O2 and GES-1. Bioassay-guided separation of the 95% ethanol extract from the roots of E. kansui led to the isolation of 12 diverse terpenoids whose structures were identified by H-1, C-13 NMR spectroscopy and ESI-MS as kansuinine A (1), kansuinine B (2), kansuinine C (3), kansuiphorin C (4), 3-O-(2'E,4'Z-decadienoyl)-20-O-acetylingenol (5), 3-O-(2'E,4'E-decadienoyl)-20-O-acetylingenol (6), 3-O-(2'E, 4'Z-decadienoyl)-20-deoxyingenol (7), 3-O-benzoyl-20-deoxyingenol (8), 5-O-benzoyl-20-deoxyingenol (9), kansenone (10), epi-kansenone (11), euphol (12). All these 12 terpernoids were evaluated in vitro for cytotoxicity on L-O2 and GES-1 cell lines. Most ingenane-type diterpenoids and 8-ene-7-one triterpenoids (5-11) exhibited a relatively lower IC50 value; therefore, these compounds had stronger cytotoxicity against human normal cell lines L-O2 and GES-1 with dose-dependent relationships. These results will be significantly helpful to reveal the mechanism of toxicity of kansui and to effectively guide safer clinical application of this herb.