Gene delivery, one important cancer-therapy mode, still remains to be challenging because of the shortage of highly efficient and safe nonviral vectors. Here, we revisit the development of cationic phosphorus dendrimers by synthesizing them with different generations (G1–3) and surface ligands (1-(2-aminoethyl) pyrrolidine, 1-(3-aminopropyl) piperidine, or 1-(2-aminoethyl) piperidine) for optimized gene delivery toward cancer-gene-therapy applications. First, the synthesized dendrimer derivatives were employed to condense plasmid DNA (pDNA) encoding enhanced green fluorescent protein (EGFP) to optimize their gene-delivery efficiency by varying the dendrimer generations and surface polycationic ligands. We show that all dendrimer/pDNA polyplexes display good cytocompatibility, and the 1-(2-aminoethyl) pyrrolidine-modified protonated G1 dendrimers (1-G1) display the best gene-delivery efficiency to HeLa cells under the same conditions through flow cytometry and fluorescence microscopic imaging analyses. Hence, 1-G1 dendrimers were then used as a vector to transfect pDNA encoding both EGFP and p53 protein for cancer-gene-therapy applications. Our results reveal that under the optimized conditions, the transfection of pDNA induces the significant p53 protein expression as verified through the resulted cell cycle arrest (regulation of p21 and Cdk4/Cyclin-D1 expression) and Western blotting. The cancer-gene-therapy potential of the polyplexes was finally validated through therapy of a xenografted tumor model after intratumoral injection without systemic toxicity. The developed cationic 1-G1 dendrimers may be adopted as a powerful vector system for gene therapy of cancer, as well as for highly effective gene therapy of other diseases.