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The detailed microstructural features of WC-Co nanocomposite coating, manufactured by high velocity oxy-fuel spraying, have been characterized by high resolution transmission electron microscopy and computer-aided image processing. The coating was built up by depositing many individual droplets that experience different thermal histories during flight, developing an inhomogeneous microstructure. The nanocomposite coating contained very high amounts of non-WC phases including W2C, W, η-carbide and amorphous Co-rich phase. The unreacted WC particles were also retained in the coating, with similar size and morphology in the feedstock powder, and had stacking faults with 1/6<1213> atomic displacement vectors. Two different types of near-spherical W2C particles were formed by either direct decomposition of WC into W2C and C or precipitation from liquid Co-W-C, which was attributed to the extent of feedstock melting and decarburization in specific droplets. The different formation mechanism yielded a significant difference in W2C particle size in the coating. Metallic W particle and η-carbide were also formed in specific locations in the coating. Rapid solidification promoted the formation of amorphous binder phase in which large amounts of W and C elements were dissolved in Co-rich matrix.