We investigate light-induced crystallization mechanism of hen egg-white lysozyme. Photochemical intermediate radical has been expected to form cluster which grows to nucleus. To confirm the nucleation process that the radical grows to the crystal, SDS-PAGE, crystallization experiments were carried out. Covalent-bonded dimer was produced by photochemical reaction, and it grows to the crystal. We conclude that the dimer plays the role of the smallest stable cluster in early stage of the crystallization process.