A block replacement policy using items with different reliability is discussed. We devide system unit failure modes into two modes and use less reliable unit when operating unit fails near the planned preventive replacement time. In this policy, item A has two failure modes. Mode-1 failure is removed by minimal repair, mode-2 failure by replacement. If mode-2 failure of item A happens in (0,T- $delta$). failure item A is replaced by new item A. If mode-2 failure of item A happens in(T-$delta$,T), failure item A is replaced by new item B. Item B should be cheaper and less durable than item A. Under this policy, we determine the preventive replacement interval T and the interval $delta$ of item B replacement which minimize the cost rate per unit time.