Distribution of microwave radiation power density on the drying bed in a typically structured microwave drying chamber was analyzed based on the antenna theory and experimental data. The results showed that, when material with a relatively large dielectric loss factor to be dried is spread fully over the drying bed in a drying chamber, which is the most common condition for large scaled microwave drying, the material on the drying bed will first absorb most of the radiation energy of the incident microwave from the mouth of the microwave guide connected with the drying chamber. The distribution of microwave radiation power density on the drying bed could be determined by analyzing the radiation pattern of the open-ended microwave guide.