Over the past few decades, metallic nanoparticles (NPs) have been of great interest due to their unique properties which distinguish them from those of bulk metals. Many attempts have been conducted to investigate the characteristics of NPs and their applications. However, the sintering process which converts metallic NPs to conductive film was not established yet. In this study, the microstructure evolution of Au NPs after sintering under different thermal condition was examined and the film quality was studied based on densification, organic residues and electrical resistivity. Au NP ink dispersed in a toluene were spin coated on Ni-plated FCCL or Si substrates and thermally treated in a furnace under different sintering profiles under various types of flows such as air, nitrogen (N2), or reducing atmosphere of formic acid (FA). The Au ink was consisted of Au NPs coated with an organic capping molecule. The capping molecules not only help NPs to disperse but prevent aggregation and precipitation of NPs out of solution. When the NPs are treated by thermal process, the surface ligands from capping molecule start to decompose and necking and melting of NPs occur producing the film with the electrical conductivity. The diameter of Au NPs was approximately between 5-7 nm with spherical shape. The Au film sintered under air showed only necking between neighboring Au NPs without further grain growth. When Au NPs films were sintered under N2 atmosphere, NPs fused together in clusters. Under sintering with flows of FA, a larger area of pores due to the volume shrinkage of the film was observed since an agglomeration and melting of NPs were considerably progressed compared to the film sintered under N2. Sintering with a flow of a single gas such as air or N2 showed organic residues in the film indicated by C-H or C-O stretch peaks. However, when mixed flows of FA and N2 were applied, there was no IR peaks from organic substances observed in the film. It is assumed that the organic capping molecules surrounding the Au NPs were removed significantly with sintering with two flows of FA and N2. The microstructure showed less pore distribution and lower level of organic residues compared to those sintered under air, N2, or FA atmospheres. The electrical resistivity was about twice of bulk value of 2.44 μΩ -cm. Overall Au NPs film sintered under FA and N2 resulted in a better sintering effect based on densification of the film and level of residual organics, translating into a relatively high electrical conductivity.