In this work, Box–Behnken design was used for investigating the influence of the main operating parameters (Argon flow rate: Ar, hydrogen flow rate: H and arc current: I) on velocity and temperature of La Ce O in-flight particle characteristics (i.e., mean velocity and temperature) during supersonic atmospheric plasma spraying (SAPS). The relationship between the particle characteristics and the composition and the microstructure of LC coating was investigated eventually. During spraying, the particle temperature and velocity were monitored by on-line diagnosis system. The results indicated that the particle velocity linearly increased with the increase of Ar, H and I. I and the interaction between I and Ar and/or H were the most important parameters which influenced the particle temperature. The content of Ce in the as-sprayed coating rapidly decreased when the particle temperature increased from 2923 to 3036 °C, and then slowly decreased in the range of 3127–3218 °C. When the particle velocity fluctuation was less than 5%, the porosity of as-sprayed coatings decreased with the increase of particle temperature, however, it increased with the increase of particle velocity.