H⁺ irradiation increases the surface hardness of polycarbonate. Nano indentation measurement shows that the hardness increases up to 3.7 GPa at the dose of 5 × 10¹⁶ # cm⁻² and at the irradiation energy of 150 keV. In addition, the hardness increases with the dose and the energy of H⁺ irradiation. In accordance with the nano indentation measurement, the Fourier-transform infrared spectroscopy (FTIR) depends on the dose and energy of H⁺ irradiation. The peak at ∼1500 cm⁻¹ for the aromatic ring and the peak at ∼1770 cm⁻¹ for the CO stretch decrease with increasing dose and energy, while the increase of the dose and energy develops a new CO stretch vibration at ∼1700 cm⁻¹ and forms aromatic hydrocarbons at ∼1600 cm⁻¹. X-ray diffraction experiments are also consistent with the nano indentation measurement and FTIR spectra. Based on the experiments, we discuss a possible mechanism of the surface hardness enhancements by ion beam irradiation.