Peptide dendrimers represent superior drug carriers for their unique nanoarchitectures, excellent degradability and biocompatibility. In this research, poly(L-glutamic acid) dendrimers with polyhedral oligomeric silsesquioxane (POSS) as cores were synthesized. Tumor targeting moiety (biotin) and therapeutic drug doxorubicin (DOX) were immobilized on the dendrimers via pH-sensitive hydrazone bonds. The size distribution and morphology of the drug-dendrimer conjugates were characterized by DLS, AFM, and TEM. The drug release profiles, cellular uptake, in vitro and in vivo anti-tumor activities of the conjugates were investigated. The results revealed that the conjugates aggregated nanoparticles with the size around 100 nm. The drug-dendrimer conjugates could be internalized in mice breast cancer 4T1 cells efficiently. The IC50 of the conjugates was comparable to that of DOX·HCl. The in vivo experiments were carried out in mice xerograft breast cancer models, the results indicated that the inhibition efficiency of the DOX-dendrimer conjugates was much better than that of free DOX·HCl.