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 GPC, H-1 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 with increasing poly(L-histidine) chain length. The morphologies of PLLA crystals changed from spherulite to dendritic crystal. 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 were characterized by DLS. AFM and TEM. The results showed that the mean diameter of mPEG(45)-PH30-PLLA(82) nanoparticles in pH = 5.0 was smaller than that in pH = 7.4. (C) 2012 Elsevier Ltd. All rights reserved.