A synthetically controllable two-step spin transition was observed in iron(II) spin crossover nanoparticles of the dehydrated one-dimensional coordination polymer [Fe(NH2trz)3]Br2 (NH2trz = 4-amino-1,2,4-triazole) using the reverse micellar method. The change from two-step to one-step hysteretic characteristics succeeded by changing the reaction time.