Abstract: IP traceback scheme has become one of the efficient coutermeasures to prevent the Denial Of Service（DOS） attack.Particularly，Probabilistic Packet Marking（PPM） first developed by Savage et al is promising and has many variants.But because of its overwritten marking and fixed marking probability，there exists the weakest link problem，which results in poor convergence of path reconstruction in PPM.A new scheme of un-overwriting PPM for IP traceback is presented.The convergent amount of marked packets requires path reconstruction and the computation overhead be both efficiently improved，and the efficiency of path reconstruction is increased.