A series of novel naphthoquinone fused cyclic α-aminophosphonates, 2-alkoxy-3, 4-dihydro-2H-naphtho[2,3-e][1,4,2] oxazaphosphinane-5, 10-dione 2-oxide 3–17 and naphthoquinone fused cyclic α-aminophosphonic monoester 18 were synthesized for the first time. These cyclic α-aminophosphonates were evaluated for antitumor activity on four human tumor cell lines, and three of them showed significant cytotoxicity (IC50: 0.019–5.15 μM) comparable to that of the reference drug doxorubicin. Furthermore, inhibition assays for topoisomerase II-mediated relaxation of supercoiled DNA indicated that the naphthoquinone fused cyclic aminophosphonates were catalytic inhibitors of topoisomerase II.