Polyurea thin films were prepared by ionization-assisted codeposition of 1,3-di(4-piperidyl)propane, and 4,4'-diphenylmethane-diisocyanate, and their optical nonlinearity was investigated in connection with the bias voltage applied on the substrate in the course of deposition. The formation of polyurea was confirmed by LR absorption spectra. At the optimum evaporation temperature for each monomer, the birefringence of the film was the largest. The film had high optical transparency in the visible range, and the absorption in the UV range was influenced by the substrate bias voltage. The film showed optical nonlinearity even without the poling process after the deposition. The Maker fringe measurement indicated that the deposited films have a peculiar dipole orientation, unlike the normal poled polymer films. The second harmonic generation characteristic was influenced by the substrate bias and also by the material of the substrate holder. These results revealed the importance of the electric field on the substrate during the deposition in determining the dipole orientation.