Abstract: The dispersion characteristics of metal periodic slow-wave structure in rectangular cross-section Cerenkov maser have been analyzed. The effects of the geometrical parameters on the dispersion curves were investigated. In order to avoid the reflect oscillation induced by the changes at two ends of the slow-wave structure, the taper for matching the smooth waveguide and the slow-wave structure was analyzed by use of equivalent circuit method. Through comparing the linear, the two-piece linear and the exponential tapers, we know that the exponential taper gives the lowest total power reflection coefficient at the end, and the total change is mild. So the exponential taper can be used as an optimal solution for transition between slow and fast modes. The effects of frequency and fabrication e