Data replication is one of the best-known strategies to achieve high levels of availability and fault tolerance, as well as minimal access times for large, distributed user communication using a worldwide Data Grid (DG). One of the challenges in data replication is to select the candidate sites where replicas should be placed and which are known as Optimal Placement of Replicas (OPR). In this paper we propose an algorithm, which is formulated by using dynamic programming method to find optimal placement k replicas of an object over DG systems, such that the overall cost (i.e. storage cost plus read cost is minimized. We have also proposed a sketch of the proof for our algorithm and its integrity.