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4-Methyl-2-phenylbenzo[de]cyclobut[i]isoquinoline-3,5(2H)-dione, C21H15NO2, resulted from the photochemical reaction of N-methyl-1,8-naphthalenedicarboximide with phenylacetylene, has been characterized by 1H NMR, IR, and MS spectra and established by single-crystal X-ray crystallography. The crystal structure belongs to the monoclinic space group P21/n with cell parameters a = 12.530(2) A, b = 9.324(2) A, c = 13.296(2) A, β = 96.85(1)○, V = 1542.3(4) A3, and Z = 4.