This paper presents a group maintenance m떨el with two modes of failures, critical and minor failures. To a fleet of machines under operation, failures occur according to a probability law. Upon minor failures, minimal repairs are performed to restore the machine to the operating condition immediately. If a critical failure occurs, the failed machine is left idle until the next group maintenance. At the group maintenance, the failed machines are replaced by new ones, and the survivals are serviced to return it to an as-good-as-new condition. A group maintenance is performed if a prescribed time interval elapses or the number of critical failures reaches to a certain amount, whichever comes first. The mean cost rate expression under the group maintenance policy is derived.