Abstract: AIM　To search for antineoplastic drugs with fewer side effects and improved activities, using tetrazine derivatives with different substituted groups in their phenyl rings. METHODS　The title compounds have been synthesized by reacting 3,6-dimethyl-1,6-dihydro-1,2,4,5-tetrazine with substituted phenyl isocyanates in the precence of p-dimethylamino pyridine as catalyst. RESULTS　Eighteen new N,N′-disubstituted phenyl-3,6-dimethyl-1,4-dihydro-1,2,4,5-tetrazine-1,4-dicarboamides have been synthesized. Their structures have been comfirmed by IR, ¹HNMR and elemental analysis. The antineoplastic activities were screened. The structure-activity relationship has also been studied. CONCLUSION　The meta-substituted tetrazine derivatives showed marked antitumor activities against P388 and A-549 when electronic effect constants and hydrophobic constants of their substitutents are in a certain range.