Abstract: We have studied the gain characteristic of a FEL and found that there is a kind of micro -FEL device with super-high gains. This FEL device lies in the far infrared range, It consists of a rflinac with a high brightness electron beam, a microwiggler, and a capillary waveguide. micro-FEL device, super-high gain, microwiggler, capillary waveguide.