High boron containing Fe-Cr alloy powder has been investigated in this research as to its microstructure evolution by the rapid solidification in thermal spraying process. The Fe-Cr alloy powder consists of Cr-rich metal borides, such as Cr_(1.65)Fe_(0.35)B_(0.96) and Cr₂B, within Fe-Cr solid solution matrix phase. In the alloy powder, the boron content in the matrix phase was extremely low. Boron was supplied to the matrix Fe-Cr solid solution phase by the thermal dissolution of Cr-rich boride particles during the thermal spraying. Cross-sectional TEM observation on the coatings indicates that the coated layer is composed of equiaxed nanocrystalline Fe-Cr solid solution phase and boride particles. The formation of equiaxed nanocrystalline phase indicates that large undercooling could be attained during the rapid solidification of thermal spraying process.