High-T-c superconducting flux flow transistors were fabricated with co-evaporated thin films of YBaCuO. The vortex flow channels (2 mu m in width) and the device patterns were formed by Ar ion milling. The three-terminal characteristics, vortex flow characteristics, transresistance, and current gain of the device were measured. The AC input-output characteristics of the device with an Au load resistor were also measured. The measured how voltage, transresistance and current gain are discussed in relation to these AC input-output measurements.