Poly(L-lactide-co-ε-caprolactone)-b-poly(L-lactide) [P(LL-co-CL)-b-PLL] diblock copolyesters were synthesized in a two-step process with 1-dodecanol (DDC) and stannous octoate as the initiating system. In the first-step reaction, a 50:50 mol % amorphous poly(L-lactide-co-ε- caprolactone) [P(LL-co-CL)] copolyester was synthesized via the bulk copolymerization of L-lactide and ε-caprolactone, which was followed by the polymerization of the PLL crystalline block at the end chain in the second-step reaction. The yielded copolyesters were characterized with dilute-solution viscometry, gel permeation chromatography, ¹H- and ¹³C-NMR, and differential scanning calorimetry methods. The molecular weights of the P(LL-co-CL) copolyesters from the first-step reaction were controlled by the DDC concentrations, whereas in the second-step reaction, the molecular weights of the P(LL-co-CL)-b-PLL diblock copolyesters depended on the starting P(LL-co-CL) copolyester molecular weights and L-lactide/prepolymer molar ratios. The starting P(LL-co-CL) copolyester molecular weights and PLL block lengths seemed to be the main factors affecting specific thermal properties, including the melting temperature (T_m), heat of melting (ΔH_m), crystallizing temperature (T_c), and heat of crystallizing (ΔH_c), of the final P(LL-co-CL)-b-PLL diblock copolyester products. T_m, ΔH_m, T_c, and ΔH_c increased when the PLL block lengths increased. However, these thermal properties of the diblock copolyesters also decreased when the P(LL-co-CL) block lengths increased.