Poly($varepsilon$-caprolactam) (PCL)-b-poly(propylene glycol) (PPG)-b-PCL type triblock copolymers with various PPG contents(0~35wt%) were synthesized by anionic ring opening polymerization of $varepsilon$-caprolactam(CL) using NaH catalyst and CL-blocked-PPG-(hexamethylene diisocyanate) prepolymer as a micro-activator. Changes of thermal properties of the block copolymers with PPC content were investigated by DSC. TGA, and structural changes by X-ray analysis. Glass transition, crystallization, and meriting temperatures decreased and the transitions became broader with increasing PPG content. Heat of crystallization and melting also decreased with PPG content, but were similar when normalized with respect to the amount of PCL block in the copolymer, i.e., non-crystal1izable PPG units affected the rate of crystallization and crystallization temperature but the extent of crystallization of PCL parts were similar regardless of PPG content. Equilibrium melting temperatures obtained from Hoffman-Weeks plots also decreased with PPG content, but the values of stability parameter were almost constant(0.38~0.40) The results and the similar values of normalized heat of fusion or crystallization indicated that the crystal morphology of PCL units in block copolymers were not affected by the presence of PPG units. The polymer-polymer interaction parameter calculated from melting point depression was -0.239 at 229.5$^{circ}C$ and the interaction energy density was -17.3211cm3. The crystalline structure of PCL units in the block copolymers is the a-form. Decomposition temperatures of the block copolymers decreased with increasing PPG content.