Abstract: The effect of annealing on the structural, optical and the photovoltaic properties of CdS:CuxS heterojunction formed by acid etching of sintered CdS layer prepared and dipping it into a cuprous ion solution were investigated for the purpose of fabricating cheap and high efficient solar cell. Each of the CdS:CuxS sample prepared was characterized using the X-ray diffractometer (XRD), UV-visible spectroscopy and the I-V measurement in dark and light condition after annealing at 200 °C, 250 °C and 300 °C. The study concluded that annealing was crucial for optimizing structural and optical properties of CdS:CuxS heterojunction which eventually affect the photovoltaic properties of the samples.