In braided channels which often form in mountain streams, sediment discharge varies spatially and temporally with the variation of channels. Results obtained from flume tests show that merging and diverging phases of channels occur alternately and repeatedly in space and time, which bring about the periodic change of sediment discharge. In order to estimate the sediment discharge averaged temporally, the average width of the channel should be known. We propose a new method predicting the width of channel in terms of the concept of the self-formed channelwhich is similar to the criterion of alternating bars formation.