The paper deals with a modified time based maintenance policy for mono-unit systems with the presence of emergency spare part ordering cost due to unplanned maintenance actions. Comparing with existing maintenance policies in the literature, in the proposed one an unplanned replacement threshold is in addition introduced to balance the emergency ordering cost and the downtime cost. It is show that the proposed policy is an extension of many popular policies. To evaluate the performance of the proposed policy, a cost model is used. Moreover, an analytical approach allowing the calculation of the maintenance cost is developed. Finally, a numerical example is proposed to show the advantages of our policy when compared to the other existing policies.