The authors present a new approach to evaluating the loss periods in the output multiplexer of ATM switch. First, they present a model for the output multiplexer using a discrete time finite capacity queueing system model with a threshold in the queue and priority in the bursty incoming cells. In modelling the loss period, they divide the queue space into a nonloss region, a half-loss region and a full-loss region, and define their corresponding time periods as the nonloss period, half-loss period and full-loss period, respectively. Next, they derive a general formula for the mean sojourn time for which a queue stays in a region. They derive formulae for the loss period, cell loss rate due to buffer overflow, discard rate for low priority cell and cell waiting time. Finally, they present the results of numerical experiments.