Novel pH-sensitive physically crosslinked hydrogels were synthesized by grafting D,L-lactic acid (LA) onto the amino groups in chitosan (CS) without a catalyst. The structures of these graft copolymers were analyzed by FTIR and X-ray measurements. Degree of substitution, side-chain length, and yield of copolymers were evaluated from H-1-NMR, salicylaldehyde, and elemental analysis. The crystallinity of chitosan gradually decreased after grafting, since the side chains substitute the -NH2 groups of chitosan randomly along the chain and destroy the regularity of packing between chitosan chains. Water uptake of the hydrogels was investigated as a function of side-chain length and degree of substitution. The influence of pH and salt concentration on the swelling behavior of the hydrogels was determined and interpreted.