Abstract: The operation power of a reactor is an important parameter to be monitoredThis report briefly describes the determing and mornitorings of maximum permis-sable power for HWRR-3. The method of calculating is described and the resultof calculation and analysis of error are given. On line calculation and real time monitoring at the heavy water reactor havebeen realized. It provides the reactor a real time and reliable supervision. Thismakes operation convenient and increases reliability.