Aqueous solutions of both poly(ethylene glycol)/poly(DL-lactic acid) (PEG/PDLLA) and poly(ethylene glycol)/poly(L-lactic acid) (PEG/PLLA) multiblock copolymers underwent "clear sol-to-turbid gel-to-syneresis" transitions as the temperature increased. To investigate the stereoisomeric effect on the phase behavior, PEG/PDLLA and PEG/PLLA multiblock copolymers with a similar block length and total molecular weight were carefully synthesized. Compared with the PEG/PDLLA multiblock copolymer, the PEG/PLLA multiblock copolymer showed a lower critical gel concentration, a lower sol-to-gel transition temperature, and a wider gel window. In addition, the maximal gel modulus of the PEG/PLLA multiblock copolymer (11.0 wt % in water) was 2 times larger than that of the PEG/PDLLA multiblock copolymer. X-ray diffraction spectra, transmission electron microscopic images, and polarizing optical microscopic images suggested the amorphous state for both PEG/PDLLA and PEG/PLLA multiblock copolymer thermogels. C-13 NMR spectroscopy, critical micelle concentrations, hydrophobic dye partition, and circular dichroism spectra suggested that the differences in the thermogel properties come from lower dynamic molecular motion and the higher aggregation tendency of the PLLA due to the isotactic localization of the hydrophobic methyl groups rather than crystallization of PLLA.