In an O type high power travelling wave tube (TWT), its power capacity is determined by the heat conduction characteristic of the slow wave structure (SWS). When the open type SWS is supported by dielectric rods of trapezoid shape in cross section, the one dimensional equation of the temperature difference between the hot spot of the SWS and the barrel of the tube is derived in this article. The value of temperature difference determines the power capacity of the TWT. The equations obtained are then applied to analyze different kinds of practical open type SWS. Theoretical calculation is in good agreement with measurement. Some values. of temperature difference for several practical SWS, some of which are made by spring pressed, and some by brazing, are given for comparision and for use in technological design and analysis.