Statistics and Analysis of Microwave Type Ⅲ Bursts

87 groups of type Ⅲ bursts recorded with the dynamic spectrograph (from 5.7 to 7.6GHz) during the 23^rd solar cycle (from August 1999 to January 2002) at the National Astronomical Observatories are statistically analysed in this article,especially with the frequency drift,duration,bandwidth and polarization of each of these events being analysed in detail.From the observational characteristics the authors hold that the microwave type Ⅲ bursts may be caused by the harmonic plasma radiation nonthermal electronic beams and the electron gyro-maser radiation.