Abstract: The influence of quartz tube on the electric field intensity in the parallel WR-430 waveguide resonant cavity is discussed. The electric field intensity has an abrupt change like a positive impulse near every slot and the impulse peak increases along the surface of one waveguide while decreases along the surface of the other when there is no quartz tube. In the presence of quartz tube, the electric field intensity gets weaker and fluctuates more irregularly along the wall of quartz tube. Noteworthily, it waves along the axis of the plasma generation zone in these two cases. While the wall thickness of quartz tube is 5 mm and the distance of quartz tube from waveguides is 2 mm, the average electric field intensity reaches maximum and the high electric field area is comparatively large. The maximum electric field intensity decreases with the increase of the wall thickness and distance for their values respectively greater than 5 and 2 mm. Low-pressure (1 500 Pa) and atmospheric air microwave plasmas are produced and their appearances are much similar to corresponding side-distributions of electric field intensity.