Positronium (Ps) is an ideal system for precision test of bound state quantum electrodynamics. The hyperfine splitting (HFS) of the ground state of Ps, which is one of the most precisely tested quantity, has a large discrepancy of 16 ppm (4.5 σ) between previous experiments and theoretical calculation up to O(α 3lnα −1) and part of O(α 3) corrections. A new experiment which reduces possible systematic uncertainties of Ps thermalization effect and nonuniformity of magnetic field was performed.
