A series of heteroleptic platinum(II) complexes (1a-3a and 1b-3b) bearing a dicarbene ligand and a cyclometalated bidentate ligand have been prepared. Two of the complexes (1a and 3a) were characterized by X-ray diffraction analyses, confirming the square-planar structures. Detailed spectroscopic properties of the complexes have been studied, and two complexes, 3a and 3b bearing a carbene chelate, which has a trimethylene group between the two N-heterocyclic carbene ligating groups, show intense luminescence (quantum yields >50%) in fluid solution at room temperature. Non-radiative rate constants of the emissive excited states of the complexes vary markedly among the 6 complexes, although radiative constants of them lie in a narrow range of values, so that the difference in the photophysical properties is ascribed to the difference in the non-radiative decay rates. The photophysical parameters have been discussed with the results of the quantum mechanical calculations. The DFT and TDDFT calculations show that the emissive excited states have mainly ligand-centered character with a slight contribution of MLCT.