In this paper, the ONO layer scaling and the leakage components in a SONOS cell have been extensively studied. The reliability with focus on both endurance and data retention has been demonstrated, Results have shown that the cell with thinner blocking oxide has better endurance, while it has poorer data retention. However, this can be achieved by a good control of the oxide quality. In terms of the data retention, thermionic and direct tunneling, in relating to the charge loss, are the two dominant leakage components, which can be separated. Moreover, after cycling, we can separate another third leakage component, the trap-to-trap tunneling induced leakage. These results are useful toward an understanding of the leakage mechanisms of SONOS cell as well as the scaling design of ONO layers.