The present invention relates to thermochromic micro particles capable of stably implementing a high chromogenic property and a decolorization property and having an improved erasing property, thermal resistance, and chemical resistance, and a thermochromic ink composition, a writing apparatus, and a smart window using the same. The thermochromic micro particles comprise a core part including a phase change material and a dye, the phase change material including a phenol-based compound containing an aliphatic functional group having a carbon number of 6 or more; and a shell layer including a polymer resin.