The objective of this study is to synthesize gelatins grafted with lactic acid oligomer (LAo) and lactic acid oligomer-poly(ethylene glycol)-lactic acid oligomer (LAo-PEG-LAo) triblock copolymers and to examine their gelation behavior. The grafting ratio of LAo-grafted gelatins could be changed by the feed ratio of LAo to the amino groups of gelatin for grafting reaction, while triblock copolymers with different molecular weights of LAo were prepared. The turbidity of LAo-grafted gelatin solution increased with the increased grafting ratio, although the turbidity increase was suppressed by adding guanidine hydrochloride in a dose-dependent manner. The turbidity of LAo-grafted gelatin solution further enhanced by mixing with the solution of triblock copolymers, while the enhanced extent increased with an increase in the LAo molecular weight. A mechanically firm hydrogel was formed by mixing the LAo-grafted gelatin and triblock copolymer at the higher solution concentrations, while the compression strength of the hydrogel became higher as the grafting ratio increased. No hydrogel formation was observed in the presence of guanidine hydrochloride.