Fluorine (F) ions which deduced from tungsten-polycide (W-polycide) formation and affected on metal-oxide-semiconductor (MOS) devices with pure oxide, oxynitride (NO) and re-oxidized oxynitride (ONO) gate dielectric under high field stress were investigated in detail. Although a significant amount of interface states were detected by charge pumping technique in NO and ONO device with W-polycide gate in comparison with that devices with polysilicon gate, however, it is found that NO, and especially ONO gate dielectric, can effectively improve the hot-carrier degradation in W-polycide gated MOS field-effect transistor (FET). This mechanism is well interpreted with a model about F ions increasing the nitrogen concentration at the interface between gate dielectric and Si substrate according to the result of secondary ion mass spectrometry (SIMS) analysis.