The sintering of Ni nanoparticle agglomerates in the gas phase is studied. The partially or completely sintered agglomerates are characterized in flight by in situ electrical mobility measurement and after their deposition by transmission electron microscopy. The mobility diameter, the mean primary particle diameter, and the total surface area of the agglomerates are determined as functions of the sintering temperature. The experimental results are analyzed using an empirical law for the primary particle coarsening as well as by means of a modified Koch-Friedlander theory. It turns out that the activation energy for the dominating diffusion process is Ea=0.6 eV∕atom, which is characteristic for surface diffusion in Ni. Our analysis provides a consistent picture with respect to both the temperature dependence of the characteristic sintering time and the mean diffusion length.