Recently, high-functioning hardware devices such as smart TVs and smart phones have been widely used in our daily lives. To keep up with the rapid advance of these high technologies, reconfigurable hardware devices such as FP-GAs (Field Programmable Gate Arrays) have been used in final products. Under the circumstances, the risks that mal-functions may be inserted into hardware devices have arisen. The malfunctions inserted into hardware devices are known as hardware Trojans. How to detect them becomes serious concern in hardware production. In this paper, we design a Trojan-infected cryptographic circuit as well as a Trojan-invalidating circuit, and implement them on an FPGA board. To begin with, we design an AES cryptographic circuit. Secondly, we insert a hardware Trojan into the AES cryptographic circuit. Finally, we design a Trojan-invalidating circuit and insert it into a suspicious Trojan net in the Trojan-infected cryptographic circuit. After that, we implement the circuits into an FPGA board. The experimental results demonstrate that the Trojan-invalidating circuit adequately deactivate the suspicious Trojan net in the Trojan-infected cryptographic circuit.