It has been said that plasma-sprayed ceramics particles are often supercooled before the impact on substrate. Some numerical models of the droplet impact actually included the supercooling effects. However, there is no report that has experimentally confirmed the effects on splat morphology. Therefore, in this research, we have mainly investigated the supercooling effects on splat morphology as well as splat microstructure. To achieve this, we developed an in-situ measurement technique utilizing radiation from a melt particle to monitor the impact of single particle successively under plasma spraying. The system was able to identify each single particle, which enabled us to correlate the splat morphology with impact velocity and thermal history of each particle during the impact. Yttria-stabilized zirconia powders were sprayed onto quartz glass substrate by the argon-hydrogen dc-rf hybrid plasma under atmospheric pressure. Waveforms of emissions and thermal history obtained during the impact were precisely analyzed. Especially, we closely examined thermal history during particle spreading to find the recalescence. In addition, splat morphologies were examined statistically in relation to their thermal histories. Based on the measurement, we also evaluated the viscosity of zirconia, cooling rate, and thermal contact resistance experimentally.