In the classical picture of the magnetotail current sheet, the current carriers are mainly ions that undergo nonadiabatic motions. Oxygen ions could contribute a thicker current sheet than the protons due to their large gyroradii. In this paper, however, we report a thin energetic O+ layer embedded in the magnetotail proton current sheet observed by Cluster on 11 October 2001. The thickness of the O+ layer is estimated to be smaller than the half thickness of the local current sheet. And the layer is formed by energetic O+ with energy larger than 20 keV moving along the +gamma direction in GSE coordinates. It is inferred that these energetic O+ are probably generated due to the selective acceleration effect of the thin reconnection electric field to the upstreaming ionospheric O+ ions with different initial condition.
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