Self-assembly behavior of mixture systems containing poly(gamma-benzyl-L-glutamate)-poly(ethylene glycol) graft (PBLG-g-PEG) and block (PBLG-b-PEG) copolymers in aqueous solution was investigated by both experiments and computer simulations. Pure graft copolymers self-assembled into vesicles, and pure block copolymers aggregated into spherical micelles or vesicles, while, for the mixture systems, hybrid cylindrical micelles were observed In addition to the experimental observations, self-consistent field theory (SCFT) simulations were performed on the self-assembly behavior of graft/block copolymer mixtures. Simulation results reproduced the morphological transitions observed in the experiments. Moreover, from the SCFT simulations, the chain distributions of copolymers in the aggregates were obtained. For the hybrid cylindrical micelles, block copolymers were found to mainly locate at the ends of aggregates, which prevents the fusion of cylinders to vesicles. By combining experimental findings with simulation results, the mechanism regarding the morphological transition of the aggregates formed by graft/block copolymer mixtures is proposed.