The crystal of (beta-cyanoethyl)bis(dimethylglyoximato)(4-methylpyridine)cobalt(III) exhibits a reversible single crystal-to-single crystal thermal phase transition at 343 K. The beta-cyanoethyl group of the crystal was isomerized to alpha-cyanoethyl group retaining the single crystal form when the crystal was irradiated with a strong Xe lamp above 343 K, although the crystal was decomposed below the temperature. On the other hand, when the crystal was exposed to a weak Xe lamp at 340 K, the crystal showed the phase transition after 20 hours exposure keeping the single crystal form. The crystal after the phase transition also exhibited the beta-alpha photoisomerization on exposure to the strong Xe lamp. The mechanism has clearly been elucidated on the basis of the analyzed structures before and after the photoisomerization or phase transition.