This study examines in vitro and in vivo biodegradation and biocompatibility of a thermogelling polymeric material, which we call a mixture hydrogel. The mixture contains two ABA-type triblock copolymers poly(d,l-lactic acid-co-glycolic acid)-b-poly(ethylene glycol)-b-poly(d,l-lactic acid-co-glycolic acid) (PLGA-PEG-PLGA) with different block ratios, and one polymer is soluble in water, but the other is not. The aqueous solutions of some mixtures with appropriate mix ratios form hydrogels at the body temperature. The degradation of mixture hydrogels proceeded by hydrolysis of ester bonds followed by the erosion of gel in phosphate saline buffer solution at 37 degrees C for nearly one month. The mass loss and reduction of molecular weight were detected. The mix ratio was found to significantly influence the degradation profiles. The rapid in vivo gel formation was confirmed after subcutaneous injection of the thermogelling copolymer mixtures into Sprague-Dawley rats. The in vivo degradation was a bit accelerated than in vitro hydrolysis, and the persistence time of injected hydrogels in vivo was found to be tuned by mix ratio. MTT assay and histological observations were used to examine the copolymer mixtures. Both in vitro and in vivo results illustrate acceptable biocompatibility of our materials. As such, the thermosensitive hydrogel of copolymer mixture is confirmed to be a promising candidate of an injectable biomaterial for drug delivery and tissue engineering.