The photo-induced phase transition (PIPT) in the spin-crossover complex, [Fe II (2-pic) 3 ]Cl 2 EtOH, has been investigated by monitoring the ESR signals of impurity Co 2+ ions. The principal values of the g-tensor of Co 2+ ions in the low temperature phase at 4K were determined as g xx =2.1, g yy =2.6 and g zz =7.1. The light irradiation resonant to the d–d transition of Fe 2+ ions (532nm) causes the change of g zz from 7.1 to 7.35. The intensity of the photo-induced signal exhibits nonlinear dependence on the irradiation time. These results suggest that the crystal field on a Co 2+ ion would approach to that of cubic symmetry by PIPT.