The diffusion of triplet exciton in double-layered crystals of benzophenone and naphthalene has been studied. The triplet excitons of naphthalene were formed by energy transfer from benzophenone triplet states excited by N2 laser-light irradiation at 77 K. The probability of the energy transfer from benzophenone to naphthalene was determined by the triplet-exciton lifetime of benzophenone. The zone-refining effect on the purity of the naphthalene crystals was considered by investigating the decay time of the sensitized phosphorescence from crystalline naphthalene.