The co-channel digital radio-relay system improves effectively the use of radio frequency allocations by transmitting signals separately on both orthogonal allocations, and thereby not only the total transmission capacity of frequency band but also its bandwidth effciency are almost doubled, compared tothe interleaved digital radio-relay system. Therefore, the study on the development and application of the con-channel scheme is of importance, for the efficient use of limited frequency bnad. In this paper, we derived the eqations for calculating fade margin as a function of path length as well as error performance in terms of the outage probability for co-channel digital radio-relay system below 10GHz. And, simulation was given to demonstrate how to apply those equations to the real situation.