Abstract: A Microstrip antenna which has many operating frequency is defined as multi-frequency characterstic of microstrip antenna.Experiments show that multi-frequency is a proper property of microstrip antenna and some experimental results are presented in the table I and table II.This multi-frequency characteristic can be observed and measured by sweep-frequency method. One example is presented in figure 3 .Direct observation multi-frequency characteristic is presented first in our paper.In this example, the microstrip antanna has ten operating frequencies, from 2000 to 7000MHzThe multi-frequency characteristic can be explained by the resonator theory of microstrip and calculated by the compound calculation method which is presented in this paper.Compound calculation method is a new method.Experiments show that the error is less than 3% when compound calculation method is used.