[en] The palladium(II) and platinum(II)complexes(where, (M(L)₂X₂), M=Pd(II), Pt(II); L=isoxazole (isox), 3, 5-dimethylisoxazole(3, 5-diMeisox), 3-methyl, 5-phenylisoxazole(3-Me, 5-Ph-isox), and 4-amino-3, 5-di-methylisoxazole (4-ADI); X=Cl, Br) with isoxazole and its derivatives were investigated on antitumor activity by MM2 and EHMO calculation. Because for all the complexes the σMO energy level (E_σ(M-X) between d_x²_-y² orbital of central metal and p_x orbital of halogen atom is less than σMO energy level E_σ(M-N) between d_x²_-y² orbital of central metal and p_x orbital of N atom, without exception. And judging, from the lower E_σ(M-X) value in trans, the bonding strength was found to be weaker in trans isomer than in cis. For the Pd(II) and Pt(II) complexes which have planar ligands, it was shown that for all the complexes dissociation of X-atom in the Pd(II) complexes is easier than that of X-atom in the Pt(II) complexes in both cis- and trans-complexes. Therefore it suggests that the easier dissociation of X^- ion has some relations with antitumor activity, and a linear equation with correlation coefficient of 0.96 was found between ΔE_σ(N-X)(E_σ(M-N) - E_σ(M-X) ) and inhibitory activity coefficient, logIA