An I/O circuit includes buffers, a storage module, accumulators, timers, and an arbiter. Each buffer corresponds to a respective virtual channel. Each buffer corresponds to a respective token bucket, and outputs a normal transmission request according to the amount of tokens and an accumulating signal. The storage module stores a lookup table including a plurality of weightings. Each accumulator corresponds to a respective buffer, accumulates a data volume according to the corresponding weighting, and outputs the accumulating signal. Each timer corresponds to a respective buffer, times waiting period after the corresponding buffer outputs the normal transmission request, and outputs a time-out transmission request when the waiting period exceeds a predetermined period. The arbiter receives the time-out transmission requests and the normal transmission requests, and selects one of the buffers from all of the time-out transmission requests and the normal transmission requests.