Abstract: In the practice of microwave diathermy, electrical penetration and thermal penetration must not be confused since they are two different concepts. The factors, such as aperture of applicators and polarization, which influence penetration but are often neglected, are indicated. According to the view of this article, the thermal penetration can be enhanced by definite method in the same condition. Experiments showed that a depth of over 1 cm can be obtained in muscle at 2450 MHz.