Unrestricted density functional calculations with spin-projection procedures have been performed for a series of in-phenylene-bridged diradicals to investigate the effects of substitution on the singlet-triplet (S-T) energy gaps and the ground-state multiplicities. Our calculations show that the introduction of electron-donating (or electron-withdrawing) substituents on 4,6-positions of the m-phenylene moiety or on the radical centers, or on both positions, generally leads to a triplet ground state, although the S-T energy gaps are smaller than that of the parent m-xylylene diradical to some extent. However, the simultaneous substitution of electron-donating and electron-withdrawing groups at m-phenylene and radical centers, and vice versa, will result in a singlet ground state or a very small positive S-T gap. A perturbative analysis based on the SOMO-SOMO energy splittings, the spatial distributions of SOMOs, and the population of the spin densities calculated for the triplet state has been presented to elucidate factors determining the S-T gap and ground-state multiplicity in studied diradicals.