The Ⅲ-Ⅴ nitrides have been nowadays successfully used as active layer in high-brightness blue and green light emitting diodes (LEDs) and lasers diodes (LDs). It is more essential to know optical properties of the GaN based semiconductor materials such as optical band gap and radiation recombination transition prior to new devices fabrication. Photoluminescence (PL) spectrum is the most common tools to be employed for studying gap states in GaN. A frequent finding is that when GaN films are exposed to super-band gap illumination, a characteristic yellow luminescence band (YLB) is often observed. Many studies have attempted to uncover the origin of the yellow luminescence in GaN films. In this paper, we present an experimental investigation of the selected GaN samples grown by metal organic vapor phase epitaxy (MOVPE), using photoluminescence (PL) spectra excited with different excitation sources. The samples studied here are nominally un-doped GaN of 1 000 nm thick grown on a 10~30 nm AlN epilayer by MOVPE. Prior to the GaN epilayer growth, a 5~10 nm thick AlN buffer layer was grown on c-plane Al₂O₃ substrate. The PL spectra of the sample are excited separately by using four different excitation light sources. The correlation of the photoluminescence spectra and excitation source of the GaN film grown by metal organic vapor phase epitaxy on sapphire substrates were investigated. By analysis of the PL spectra, it has been found that the yellow luminescence band appears when continuous wave Xe lamp and He-Cd laser sources are used. The central wavelength of wider YLB is located at near 550 nm and the main peak of the band edge emission is observed at 365 nm. The YLB tends to disappear when, He-Cd and YAG pulse wave sources are employed. This phenomenon was attributed to impurity band saturation when the samples were excited by the pulse wave light sources with the high power density. It was concluded that the yellow radiation evidently depends on the excited light source.