Photoreflectance (PR) spectra of a GaN thin film and an AlGaN/GaN heterostructure were measured by using a HeCd laser or a mercury lamp as a pump beam. The wavelengths (λ) of the HeCd laser and the mercury lamp are 325 nm and 253.7 nm, respectively. The energy of the HeCd laser is lower than band-gap energy of AlxGa1-xN (x > 0.2), so electron-hole pairs cannot be generated in the AlGaN layer. Hence, the PR of the AlGaN was measured by using Argon ion laser (λ =300 nm) or quadrupled Nd:YAG (λ =266 nm) rather than HeCd laser in the previous works. In this work, the mercury lamp was used as the pump beam. Although the mercury lamp is a diffused source, it is not a hindrance to the PR measurements. The signal to noise ratio is improved by using defocused pump and probe beams in the PR measurement.