Magnetic interactions in crystal of p-cyanophenyl nitronyl nitroxide (p-CNPNN) were theoretically investigated. Semiempirical and ab initio molecular orbital calculations were performed for pair models of p-CNPNN molecules in the crystal. Effective exchange integrals (J(ab)) are positive for the pairs of neighboring molecules in a sheet parallel to the ac-plane of the crystal, suggesting pseudo-two-dimensional ferromagnetic interaction in the ac-plane. Close contact between p-CN-phenyl group and NO group of the adjacent molecules plays a dominant role for the ferromagnetic interaction. Inter-sheet interaction is weak and the sign of J(ab) depends on the pairs.