Abstract: The paper discusses the method to determine the pass--band performance of thedisk--loaded waveguide with two couplers. The fundamental agreement is foundbetween computed and experimental pass--band performances for several existingaccelerating structures. It is possible by the method to estimate the pass--bandperformance of the structure and the effects of the variation in the dimensionson its pass--band before building it. On the other hand, a way to improve thepass--band performance of the structure is pointed out.