Polyaspartamide (PASPAM) derivatives grafted with 1-(3-aminopropyl)imidazole (API), O-(2-aminoethyl)-O′-methylpolyethylene glycol (MPEG), and octadecylamine (C18) groups were synthesized and their pH-sensitive structure and Paclitaxel (PTX) load/release properties were investigated. C18/MPEG/API-g-PASPAMs systems synthesized showed a strong pH-dependent phase transition behavior near pH 6.7. Large amount of PTX up to 60–75%, depending on polymer composition, was possibly loaded into the C18/MPEG/API-g-PASPAMs nano-aggregates using a solvent-free protocol. Its pH dependent release pattern was affected correspondingly by the phase transition behavior associated with the composition of graft substituents. The pure C18/MPEG/API-g-PASPAMs systems did not show cell toxicity but the PTX-loaded copolymer systems showed a similar cell toxicity to a Taxol-type PTX. From the in vivo animal study, PTX-loaded nano-aggregates showed the much improved inhibition effect on tumor growth compared to the conventional PTX formulation.