This study was related to a solvent-free gas phase epoxidation of propylene with H2O2 vapor to synthesize propylene oxide (G-HPPO). Focus was given to the hydrothermal modification of TS-1 with NaOH-TPABr. Results showed that properly modified TS-1 exhibited 1.70 kg(PO) kg(Ts-1) h(-1) propylene oxide productivity, 93.6% PO selectivity, and 12.9% propene conversion. More than 60 wt % H2O2 utility were achieved with a 4.7 propylene/H2O2 molar ratio. Catalyst characterizations indicated that considerable amount sodium ions were introduced into the modified TS-1. The infrared absorption feature of framework Ti was shifted to higher wavenumbers. The blueshift and the NH4+-exchange experiment suggested that the modification changed the local environment of framework Ti by transforming them into "open" sites which had titanium hydroxyl groups and sodium ions on their neighboring silicon hydroxyls as counter cations. Unlike liquid-phase epoxidation, G-HPPO can be significantly benefit by the presence of the sodium ions in TS-1.