Myung Bock Kong ` Il Gwang ParkDepartment of Industrial Engineering, University of UlsanThis paper discusses condition based preventive replacement for deteriorating systems. The system continuously deteriorates in time and fails at any deterioration level which is always monitored. It is replaced at failure or at some deteriorated level preventively before failure. The deterioration process is represented by a Weibull distribution with a time-linear scale parameter. The cost rate function is formed considering replacement cost and opportunity loss cost and deterioration dependent failure distribution. If the system has an increasing deterioration dependent failure rate, the optimal deterioration level for preventive replacement can be determined from minimizing the cost rate. An illustrative example is given for a Weibull deterioration dependent failure distribution.Keywords: Deteriorating Systems, Weibull Distribution, Inverse Weibull Distribution, Deterioration Dependent Failure Rate