Network interference disturbs the legitimate receiver as well as eavesdroppers. Artificial noise transmission is able to indirectly control these negative and positive effects of interfering signals on secrecy communication, and thus improves the secrecy transmission rate. Contrary to the previous results presuming some prior information on eavesdroppers, this letter studies secrecy enhancement via the optimal use of an artificial noise without the knowledge related to eavesdroppers. We derive the optimal use of artificial noise with constraints for reliability and secrecy level and the corresponding secrecy transmission rate in the presence of network interference for the worst scenario.