[en] We investigated applicability of solid phosphor nanoparticles to radioactivity measurements by using a liquid scintillation counter. Stable water-suspendable ZnS:Cu,Cl phosphor nanoparticles was prepared to follow the literature. The prepared ZnS:Cu,Cl was dispersed into tritiated water, and its radioluminescence was counted by photomultiplier tube. The ZnS:Cu,Cl nanoparticles emitted photon by irradiation of tritium β-ray in tritiated water. The number of emitted photons increased proportionally with the tritium concentration in the sample, indicating that the tritium concentration in solution could be measured by using ZnS:Cu,Cl nanoparticles. The photon counting through the universal coincident circuit was also performed in order to reduce background noise. The lower detection limit was not improved by using the universal coincident circuit, while the emitted photons were counted for either photomultiplier. The result suggested that the concentration of ZnS:Cu,Cl nanoparticles should be optimized. (author)