CdS is an important semiconductor used in optoelectronic devices. Simple techniques for growing CdS nanostructures are thus essential at a low cost. This study presents a novel method for growing singlecrystal n-type CdS nanowires on p-type CdTe films by thermal annealing in an H 2 S/N 2 mixed gas flow, which does not require the help of a catalyst or template. The formation process and growth mechanism of the nanowires are investigated. Well-dispersed whiskerlike CdS nanostructures are obtained at an appropriate annealing temperature and duration. We suggest that the stress-driving mechanism of nanowire formation may contribute to the growth of CdS nanowires, and that the evaporation of Te through the boundaries of the CdS grain seeds plays an important role in the sustainable growth of nanowire. In addition, CdS/CdTe heterojunction device is fabricated on Mo glass. The I-V characteristic of the heterojunction in dark shows typical rectifying diode behavior. The turn-on voltage can be regulated by annealing conditions. Meanwhile, the obvious photovoltaic effect is obtained on the in situ growth heterojunction prepared at low annealing temperature. Hence, this is a new fabricated method for CdTe-based materials in the field of energy conversion.CdS is a well-studied semiconductor with versatile properties, such as a direct band gap of 2.4 eV at room temperature, a high refractive index, excellent transport properties and good chemical and thermal stability. It has been used extensively in photovoltaic devices, light-emitting diodes in flat panel displays, transistors, and logic gates 1-3 . Low-dimensional CdS nanostructures, such as nanorods, nanowires, and nanotubes, have attracted increasing attention because of their superior optical, physical, electronic and catalytic properties compared with traditional thin film and bulk materials [4][5][6] . A wide range of applications for nanodevices and nanosystems based on these CdS nanostructures has recently been explored, including a thin film transistor fabricated on CdS nanoribbons 4 , and chemical vapor deposition (CVD) [15][16][17] . Although each of these methods has its own advantages, most involve a solution process or organic solvents 13,18 , and the reaction process is uncontrollable. The CVD method is known to be effective in the large-scale fabrication of CdS nanostructures. However, due to the high sublimation temperature of CdS powder, the growth temperature of CdS nanostructures is as high as 800 °C 12,19 , and the growth process requires the use of an Au catalyst 20 . Unfortunately, Au functions as an impurity, trapping electrons and holes in the nanostructure, and affecting the performance of the device. Templates, such as anodic aluminum oxide 9,21 , which are used to prepare nanowires, can also yield better CdS arrays. However, this method also needs Au catalyst to grow nanostructure 9 . So, an effective method