In this paper, we propose an efficient per-Trunk interworking mechanism between PSTN and ATM network assuming the situation ATM network interworks with PSTN during the evolution period. The proposed mechanism improves the cell payload utilization by mapping only the active charnnels of PSTN frame into ATM cell payload. Also, we propose the frame recovery mechanism to guarantee the frame sequence integrity. The proposed mechanism is compared with other possible ones in terms of cell payload utilization, cell packetization delay. We present the implementation structure of the interworking unit. the correctness of the mechanism and the feasibility of the implementation are verifid through the CAD simulation.