The nucleus is the final target of many first-line chemotherapeutics, but the need to overcome multiple physiological barriers imposes conflicting requirements for size and charge on systemically administered drug delivery systems. Here, an N-(2-hydroxypropyl) methacrylamide (HPMA) polymer-based nanovehicle (PNV) that self-assembles from anionic HPMA copolymers with charge-reversal ability and cationic HPMA copolymers with intracellularly detachable subgroups (IDS) is described. The IDS, bearing an anticancer drug and nuclear-homing cell-penetrating peptide (R8NLS ligand), is grafted onto the HPMA copolymer via hydrazone linkage. The large, neutrally charged, self-assembled PNV (approximate to 55 nm) shows good blood persistence and preferential tumor accumulation. After tumoral arrival, the extracellular milieu actuates the disassembly of PNV to linear conjugates (approximate to 10 nm/39 kDa). This first-stage size reduction exposes R8NLS and allows for deeper tissue penetration and greater cellular internalization. After endocytosis, a second-stage size reduction occurs when the more acidic endolysosomal pH cleaved the approximate to 2.4 kDa IDS off the HPMA copolymer backbone and guaranteed the successful nuclear entry via nuclear localization signal assistance. Based on the stepwise size reduction and on-demand R8NLS exposure, the PNV inhibits growth of HeLa tumors in nude mice by 75%. This work gives important insights into the design of systemic nuclear-targeted delivery via a multistage size/charge changing way.