An endophytic fungus having antitumor activity was isolated from Viscum Coloratum of Chinese. On the basis of its morphological and 18S rDNA sequence, the fungus was identified as Lasiodiplodia theobromae and designated as A33. The broth of A33 showed effectively antitumor acitivity. In addition, TNF mRNA exoression was upregulated after treatment with A33 for 6h compared to the untreated group. Isolation of such a fungus may provide a promising alterative approach to produce antitumor agent, and A33 could serve as a potential material for fungus engineering to improve antitumor agent production.