Ion implantation of nitrogen into n-type cadmium sulfide has been investigated with energy up to 200 keV at dose levels 1015–1017 cm−2. The measurements of I-V, C-V characteristics, and photoelectric properties of diodes made on implanted CdS have given evidence for p-type layer formation. EPR studies have clarified the nitrogen center and lattice-defect centers produced by the ion implantation. Current of the implanted diodes can be expressed by I=I0[exp(eV/nkT) −1] and the parameter n was remarkably reduced from 10 to 2.7 by a 10-min anneal at 400°C. Overannealing degraded the diode characteristics. The peculiar C-V characteristics and the negative photovoltage were observed for the implanted diode, which can be understood only by presuming a series structure of the p-n junction formed by nitrogen implantation and a Schottky barrier on a p-type surface. The relative spectral response of photovoltage of the diode had a peak at 800 mμ, i.e., at 0.85 eV below the band edge after annealing below 400°C. The 800-mμ peak is reasonably ascribed to implanted nitrogen ions, because of the absence of the peak in both unimplanted and neon-implanted CdS. Optical quenching level due to radiation damage produced by implantation was also observed for photocurrent of heavily implanted samples. EPR measurements assured the existence of nitrogen centers in implanted CdS. A new EPR spectrum led to the model of N22− molecular-ion center substitutional for sulfur ion. The center was observed after annealing at 400°C, while the density of lattice-defect centers decreased.