The effect of applied magnetic field on the plane wave attenuation by microwave plasma plume inside a closed and evacuated quartz vessel is studied though spatially propagating wave method. The objective is to analyze how the operating parameters affect the wave attenuation in plasma and to find the mechanism which controls the interaction between plasma and plane wave. The experimental results show that under the present experimenal condition and in the cases without and with applied field, collisions are the reason leading to plane wave attenuation in the plasma. Plasma resonance absorption and upper hybrid resonant absorption mechanisms can not be found. To increase the plane wave attenuation in magnetized plasma, magnetic flux density should be set to make the wave test frequency and upper hybrid frequency identical.