In this work, a dual beam mode-mismatched thermal lens method was utilized in gold nanofluid containing gold nanoparticles in polyvinylpyrrolidone solution to study the effect of nanoparticle concentration on thermal diffusivity. The different concentrations of gold nanoparticles were prepared by irradiating the solution of hydrogen tetrachloroaurate (III) hydrate, premion (metals basis), in polyvinylpyrrolidone with respective dose of gamma-radiation. The average sizes of particle in the prepared samples were measured using nanophox machine. The thermal lens measurement was carried out by using a diode laser of wavelength 532 nm and a He–Ne laser for the excitation source and the probe beam, respectively. The obtained result showed an increase in the thermal diffusivity of nanofluid with the increase in concentration and particle size.