Leaky Bucket based traffic parameters are widely used for traffic declaration and enforcing in an ATM network. In this paper, we investigate the characteristics of the system that every traffic source is policed by a dual leaky bucket before entering the network. In addition to mean cell rate, peak cell rate of traffic is also taken into consideration. We find the worst output pattern from the dual leaky bucket and derive the performance bound of maximum cell loss ratio encountered in the multiplexer. It is obtained as every source transmits cells according to the criteria for extreme synchronous transmission in a coincident token-generating condition.