This paper presents parametric results of a 40 GHz Gunn-Varactor waveguide tunable oscillator. A close look at the VCO shows the complexity of the waveguide structure. Analysis reveals the existence of four inter-related radial lines. The VCO tuning characteristics, both the tuning range and the tuning power, are a function of many parameters, such as: the varactor bias and its characteristics, the Gunn bias and its characteristics, each radial line disc diameter and position (a very critical element) , and the sliding short circuit distance, etc. Almost any desired bandwidth, centre frequency, and power of oscillation can be tailored to the requirements. This analysis provides a design tool for millemeter waveguide VCOs.