The paper introduces a Fuzzy controller by in-system programmable logic circuit. The fuzzy transformation of input accurate data, Fuzzy control rules based on observing the control action and the user's experience and Fuzzy decision of output can be carried out by the programmable device. Owing to using VHDL language to write Fuzzy rules, the characteristic of this controller is the Fuzzy rules can be modified at any moment even if at work. The paper gives the controller's working principle and the source code of VHDL.