A light-induced excited spin state trapping (LIESST) experiment for a thermal gradual spin crossover complex, Fetris (2-pyridylmethyl) amine(NCS)(2) or Fe(tpa) (NCS)(2) was attempted for the first time. The high spin (HS) state after light inducement stayed metastable over a period of days without relaxation at 10 K. Intersystem relaxation from a high to a low spin (LS) complex occurred at 50 K after bleaching at 10 K. Investigation of the Mossbauer spectra of the LIESST and relaxation experiment indicated that the Debye Waller factor was a correlation parameter of the HS fraction and that the co-operative effect played a role in the relaxation process for such a solid compound.