Cytotoxic assay guided multistep separation on the dichloromethane extract of the roots of Euphorbia kansui resulted in the isolation of 10 ingenol-type diterpenoids (1-4 and 7-12), of which, 5-O-(2'E,4'E-decadienoyl)-20-O-acetylingenol (1) is a new compound, and two are jatrophane-type diterpenoids (13-14). Interconversion of two pairs of positional ester isomers (1-4) in aqueous alcoholic solution was observed, the transesterification mechanism of which was speculated and confirmed by acylation of 3 and 4 to 6 and 5, respectively. All the isolates and the two acyl derivatives (5 and 6) were evaluated in vitro for their cytotoxicities in Bel-7402, Bel-7402/5FU, BGC-823 and SGC-7901 cell lines. The 12 ingenol-type diterpenoids exhibited weak to moderate cytotoxicities, whereas the two jatrophane-type diterpenoids displayed no antiproliferative effects, which, however, may increase the antitumour efficacy of those ingenol-type diterpenoids. The structure--activity relationships were investigated by principal component analysis (PCA) of the pIC₅₀ (-logIC₅₀) values of the compounds tested and their calculated molecular descriptors. The pIC₅₀ values were highly correlated with most descriptors, especially the highest occupied molecular orbital energy (E(HOMO)), absolute hardness (η) and positively charged solvent accessible surface areas (P-ASA). As the values of E(HOMO) increase, η and P-ASA decrease, and the antiproliferative effects of these compounds increase.