[en] We investigated the surface potential around nanotube channel during CNT-FET operation by scanning Kelvin probe microscopy (SKPM). The results demonstrate that the local surface potential distribution depends on the fabrication process of FET devices. We also measured the transfer characteristic of CNT-FET and compared with the surface potential image. In addition, we investigate the specific FET device with the closed CNT loops into which channel CNT penetrates. The surface potential distribution is completely different with that of a simple single CNT channel. We find that the closed loop of CNT has a capability of trapping the charge inside the loop