We study Cu2O/InGaN heterojunction thin films with different thicknesses of Cu2O layer as a photoanode in photoelectrochemical (PEC) water splitting cell. Results show that the bandgap energy of Cu2O/InGaN heterojunction thin films is 2.60-2.72 eV and, according to Vegard's law, the indium content of the InGaN thin film is 22%. Electrochemical impedance spectroscopy shows the charge-transfer resistance value of about 0.4 k Omega for the optimized sample revealing enhanced charge separation and transfer at the interface. A maximum photocurrent density of 0.16 mA cm(-2 )at 0.5 V vs. Ag/AgCl was obtained for the Cu2O/InGaN heterojunction thin films with an overall thickness of 250 nm. The obtained value is 4.2 and 3.2 times higher than that of pure InGaN and Cu2O thin films photoanodes, respectively. We showed charge separation mechanism in the Cu2O/InGaN heterojunction photoelectrodes. According to our model, gradient energy bandgap reduce the recombination rate of photo-induced electron-hole pairs, and significantly enhance the PEC performance. (C) 2020 Elsevier Ltd. All rights reserved.