The present invention provides a novel diterpene compounds with antineoplastic activities. The novel diterpene compounds of the present invention are represented by the general formula (I). R1 to R6 in the formula represent an aliphatic group, or a radical represented by the general formula RCO—, wherein R represents an aliphatic, an aromatic or a heteroaromatic group. The compounds of the present invention may be prepared by extracting roots of Euphorbia kansui L. with an organic solvent such as chloroform, ethyl acetate and butanol. These extracted compounds may be used as starting materials for the preparation of the compounds represented by the general formula (I). The compounds of the present invention have antineoplastic activities, and therefore, useful as antineoplastic agents.