This study investigates the morphological modification of gold nanoparticles using ultrashort laser pulses and subsequent changes in the resonance spectra of localized surface plasmon polaritons (LSPPs) known as phase transition. A thin film comprising a random array of gold nanoparticles is prepared using chemically synthesized colloidal gold nanoparticles. When the femtosecond laser pulse is irradiated on the film surface, changes are observed in the microscopic image and the extinction spectrum before and after laser irradiation. The measurement results show that the femtosecond laser pulses induce the melting and aggregation of the gold nanoparticles in some microscopic areas, leading to a morphological change of the gold nanoparticles and a red-shift in the plasmonic resonance spectra.