The radiation-induced solid-state polymerization of acenaphthylene was carried out under vacuum at room temperature. The monomer and obtained polymer samples were investigated by UV, FTIR, DSC, TG, and powder X-ray diffraction methods to characterize the polymer and elucidate the polymerization mechanism. The polymer samples were crystalline with melting points ranging in 380-390 degrees C interval. Polymerization takes place through vinyl groups by a radical mechanism and crystal structure of monomer and polymer studied by powder X-ray diffraction were quite similar. The space group for both were P22(1)2 and cell parameters: a = 784.2 (6), b = 798.1(6), c = 1417.0(1) pm for monomer, and a = 791.7(8), b = 803.8(7), c = 1431.0(1) pm for polymer. The similarity of crystal structures shows a topotactic polymerization of monomer.