We are requesting 5 days of beamtime on HRPD to measure powder diffraction patterns of KOs2O6 under He gas high pressure up to 150 MPa. KOs2O6 exhibits a novel phase transition possibly associated with rattling motions of K ions at 7.6 K which is below its superconducting temperature of 9.6 K. The purpose of this study is to elucidate the mechanism of the transition through the pressure dependence of the transition. It is expected from the Clausius-Clapeyron equation that the transition vanishes at a critical point or zero Kelvin below 100 MPa.