Abstract: A theoretical investigation of the principle of self-focusing and defocusing of narrow radio beam in lower ionosphere was presented. Nonlinear propagation equation, focusing parameter and defocusing parameter are deduced, which are used to analyze the microwave beam’s width when propagating in lower ionosphere. Finally, the influence of frequency and microwave power on the final beam width is discussed. Results show that the lower ionosphere is mainly a defocusing material that will greatly reduce the microwave energy arriving at the destination, especially when the frequency is low and the radiating energy is high. But with frequency higher than several GHz, the defocusing is so weak that it can be ignored.