Frame relay (FR) service via an FR interface over a common asynchronous transfer mode (ATM) backbone network requires an FR-to-ATM interworking unit (IWU), the functionality of which directly impacts the engineering of the overall network in terms of capacity required, quality of service (QoS), and the interference among different types of services. This paper presents performance analysis of frame relay traffic in IWU. Additionally frame relay traffic policing using leaky bucket algorithm in this traffic model is carried out through computer simulation. Three measurements, the percentage of frames accepted as committed, the percentage of frames that accepted but marked eligible for diacard, and the percentage of frames that accepted that are discarded because of insufficient number of credits, are simulated variously in this paper.