The effects of tensile stress induced by SiO2 passivation layer on AlxGa1-xN/GaN heterostructure photodiode are investigated. The photodiode, with SiO2 layer annealed at 650°C for 30min in O2, shows that reverse current has decreased to 6.16nA/cm-2 under -10V, two orders lower than that of the device without annealing technique. The responsivity also increases to 0.212A/W at zero bias. The high-resolution x-ray diffraction, Hall measurements are taken to investigate the surface strain and electrical properties of p-AlGaN surface. These observations indicate that tensile stress induced by SiO2 annealing technique can improve performances of this photodiode greatly.