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Utilizing metal nanoparticles (NPs) in Additive Manufacturing (AM) enables fabricating parts with submicrometer resolution. The thermal properties of metal NPs are drastically different from their bulk and micronsize counterparts due to nanoscale thermal transport effects, e.g. ballistic phonon/electron transport instead of diffusive transport described by Fourier’s Law. Rough estimation of metal NPs’ thermal properties with bulk values will inevitably cause large errors for AM applications, because thermal properties evolve along with the sintering process. In this study, thermal properties of 100 nm Cu NPs are examined at different sintering stages. Effective density is measured between 3500 and 5300 kg/m 3 at a sintering temperature range of 100 and 400 °C, and the sintering of Cu NPs is determined to be around 300 °C using Thermogravimetry analysis (TGA) with Differential Scanning Calorimeter (DSC). A picosecond Transient Thermoreflectance (ps-TTR) technique is employed to measure the effective thermal conductivity of Cu NPs, which jumps from 18.5 ± 0.8 W/m ∙K to 26.8 ± 2.1 W/m ⋅K onset of sintering around 300 °C. These values are less than 1/10 of the bulk value (398 W/m ⋅K). The effective thermal conductivity is almost independent on porosity except in the temperature range close to 300 °C, which comes from two factors related with nanoscale thermal transport: (i) ballistic electron transport is important in particles with size comparable with electron mean free path; (ii) effective thermal conductivity is dominated by interface scattering on particles surfaces. Our results provide insights about the importance on accurate characterization of thermal properties in metal nanoparticles due to the nanoscale phenomena.