The reactions of polylactic acid (PLA) oligomers and isocyanates (4,4'- diphenylmethane diisocyanate and toluene diisocyanate) are reported. The effects of the reaction conditions, that is, the reaction time, reaction temperature, molar ratios, isocyanates, and catalyst, on the number-average molecular weight (M-n) are demonstrated. The optimum reaction conditions are determined by the synthesis of relatively high M-n PLA-based linear polyurethanes. The structure of the polymer samples was investigated with dynamic light scattering, H-1 NMR, IR, and matrix-assisted laser-desorption ionization time-of-flight mass spectroscopy (MALDI-TOF MS).