Live-streaming applications based on overlay multicast have become very popular recently. There have been many studies on optimizing the overlay construction and improving multicast performance. Most of these studies focused on building period of overlay network or multicast tree. It is not flexible in the heterogeneous and variable Internet environment. In this paper, we present DOMT, an overlay multicast protocol in which the multicast tree can be optimized dynamically. In DOMT, high capability peers get better position in the tree and more peers get near peers as their parents. Most of the previous protocols didn?t consider dynamic optimization because its bad impact to stability. In DOMT, we design some mechanisms to avoid the bad impact. The experimental results show that the optimization can decrease the height of multicast tree and shorten the average source-to-end delay. It also proves that the bad impact made by the optimization is acceptable.