It is of tremendous interest to apply laser to process nanoparticles-reinforced metals for widespread applications. However, little fundamental understanding has been obtained on the underlining physics of laser interactions with nanoparticles-reinforced metals. In this work, fundamental study was carried out to understand the effects of nanoparticles on the optical and thermophysical properties of the base metal, the corresponding heat transfer and melt pool flow processes, and the consequent surface property in laser melting. Part I presents both experimental and theoretical results on the effects of nanoparticles on the optical reflectivity, specific heat, and thermal conductivity. Electrocodeposition was used to produce nickel samples with nanoparticles. Using a power meter, the reflectivity of Ni/Al2O3 (1.8 vol. %) was measured to be 65.8% while pure Ni was at 67.4%, indicating that the Al2O3 nanoparticles did not change the reflectivity substantially. Differential scanning calorimetry was used to determine the heat capacity of the nanocomposites. The specific heat capacities of the Ni/Al2O3 (4.4 vol. %) and Ni/SiC (3.6 vol. %) at room temperature were 0.424 ± 0.013 J/g K and 0.423 ± 0.014 J/g K, respectively, close to that of pure Ni, 0.424 ± 0.008 J/g K. An experimental setup was developed to measure thermal conductivity based on the laser flash method. The thermal conductivities of these Ni/Al2O3 and Ni/SiC nanocomposites at room temperature were 84.1 ± 3.4 W/m K and 87.3 ± 3.4 W/m K, respectively, less than that of pure Ni, 91.7 ± 2.8 W/m K. Theoretical models based on the effective medium approximation theory were also used to predict the heat capacity and thermal conductivity of the nanoparticles-reinforced nickel. The theoretical results match well with the measurements. The knowledge of the optical and thermophysical properties of nanoparticles-reinforced metals would provide valuable insights to understand and control laser processing of metal matrix nanocomposites.