Euphorbia kansui is a commonly used traditional Chinese medicine for the treatment of edema, pleural effusion, and asthma, etc. According to the previous researches, terpenoids in E. kansui possess various biological activities, e.g., anti-virus, anti-allergy, antitumor effects. In this work, twenty five terpenoids were isolated from E. kansui, including thirteen ingenane- and eight jatrophane-type diterpenoids (with two new compounds, kansuinin P and Q) and four triterpenoids. Eighteen of them were analyzed by MTS assay for in vitro anticancer activity in five human cancer cell lines. Structure-activity relationship for 12 ingenane-type diterpenoids in colorectal cancer Colo205 cells were preliminary studied. Significant anti-proliferation activities were observed in human melanoma cells breast cancer MDA-MB-435 cells and Colo205 cells. More than half of the isolated ingenane-type diterpenoids showed inhibitory activities in MDA-MB-435 cells. Eight ingenane- and one jatrophane-type diterpenoids possessed much lower IC50 values in MDA-MB-435 cells than positive control staurosporine. Preliminary structure-activity relationship analysis showed that substituent on position 20 was important for the activity of ingenane-type diterpenoids in Colo205 cells and substituent on position 3 contributed more significant biological activity of the compounds than that on position 5 in both MDA-MB-435 and Colo205 cells.