Microwave has been widely used in the extraction of many samples for its unique heating mechanism, short extraction time and high yield of extract. The microstructure characteristics of plant materials are closely related with macro-property and mechanism on mass transfer within matrix. Fractal geometry, with the ability to describe irregular objects provides a new language. Astragalus slice is a special and typical plant porous media. We characterize the microstructure of Astragalus slices irradiated by microwave at 600 and 900 W by using fractal dimension, with the aim to analyze the effect of the microwave power on the extraction yield and discuss further the relationship between the fractal dimensions and microstructure changes of sample during microwave assisted extraction process. It is found that the fractal scaling law of box counting method is not suitable for the apertures on the wall of trachea inside matrix, and Slit Island and mercury injection method may be used. Fractal dimensions of samples at microwave 600 W are smaller than that at 900 W. The smaller fractal dimension is in favor of effective components dissolution inside the slices, which led to extract with the 20 min irradiation of microwave at 600 W is higher than that at 900 W.

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