The characteristics of polysilicon-oxide-nitride-oxidesilicon (SONOS) devices with different tunnel oxides are studied. The tunnel oxide fabricated by High-Temperature Oxide (HTO) with additional NO annealing treatment (HTO (NO*)) has better performance than that fabricated by HTO only and In-Situ Steam Generated oxide (ISSG) including operation window, retention, and endurance. Besides, the properties of charge-to-breakdown are also observed. The study can provide a straightforward way of reliability for future Flash memory application.