There are two traditional techniques for the realization of survivor memory unit in Viterbi decoder-register exchange（RE）and trace back（TB）method.RE is not practical for the Viterbi decoder with large number of trellis states dut to the very complicated interconnections.TB needs a large quantity of buffers and has long decoding delay.In this paper a novel technique-register/three-state-gate trace back method is presented

which combines the advantages of both RE and TB and overcomes their disadvantages.It’s very suitable for VLSI implementation.