Morphological and molecular structural effects of plasmonic heating on the Au-decorated P25 TiO2 NP porous film have been studied. With the increase of the power of a laser whose wavelength is in resonance with the LSPR of Au NPs, the phase of P25 transformed from mainly anatase phase to a rutile phase, accompanied by the formation of cracks and aggregates. At higher laser power, agglomerations of melted TiO2 were observed, indicating that the matrix surrounding the Au NPs reached a temperature as high as the melting temperature of TiO2, 1830 ºC. The irradiation at the off-resonance laser wavelength did not result in phase transition and the melting of TiO2, which confirms the role of plasmonic heating. The calculation shows that collective heating dominates the contribution to the melting of TiO2. However, the temperature measured using Raman spectroscopy was much lower than the transient temperature that Au and TiO2 NPs experienced. The results suggested that optical measurement often misses the high transient temperature that the Au and TiO2 experienced in the photocatalytic reactions.