In this paper, amphiphilic biodegradable methoxy-poly(ethylene glycol)-poly(L- histidine)-poly(L-lactide) (mPEG-PH-PLLA) triblock copolymers with pH sensitivity were synthesized. The properties of mPEG-PH-PLLA triblock copolymers were investigated by ¹H NMR, DSC, TGA, XRD and polarized optical microscopy. The results showed that the thermal properties of the triblock copolymers varied with the chain length of PH blocks. The glass transition temperatures (Tg) of the triblock copolymers increased while the crystallinities of PLLA blocks decreased with increasing poly(L-histidine) chain length. The morphologies of PLLA crystals changed from spherulite in PLLA homopolymers to dendritic in triblock copolymers. Moreover, the crystallization rate of triblock copolymers was faster than that of PLLA homopolymer. The pH sensitivity of the self-assembled mPEG-PH-PLLA nanoparticles was investigated. The mean diameter and morphology of the nanoparticles wers characterized by DLS, AFM and TEM. The results showed that the mean diameter of mPEG₄₅-PH₃₀-PLLA₈₂ nanoparticles in pH=5.0 was smaller than that in pH=7.4.