Using microwave network theory, this paper presents an analysis of open resonators in the form of waveguide with slowly-varying cross-section. A general formula for determining the trasmission Q value of the open resonator with any longitudinal profile is given. The value of Q resulting from the lossy wall is estimated. The correction of the resonant frequencies is discussed. As examples we derived the formulas to calculate the Q value for dual-cone open resonators and that for cylindrical open resonators.