GaAs1-xNx epilayers were grown on a GaAs(001) substrate by metalorganic chemical vapor deposition. Composition was determined by high resolution X-ray diffraction. Band gap was measured from 77 to 400K by using photocurrent measurements. The photocurrent spectra show clear near-band-edge peak and their peak energies drastically decrease with increasing nitrogen composition due to band gap bowing in the GaAs1-xNx epilayers. Those red shifts were particularly notable for low nitrogen compositions. However, the shifts tended to saturate when the nitrogen composition become higher than 0.98%. When the nitrogen composition is in the range 1.68-3.11%, the measured temperature dependence of the energy band gap was nicely fitted. However, the properties for the nitrogen composition range 0.31-0.98% could not be fitted with a single fitting model. This result indicates that the bowing parameter reaches 25.39 eV for low nitrogen incorporation (x = 0.31%), and decreases with increasing nitrogen composition. (C) 2003 Elsevier B.V. All rights reserved.