It is a key technology to measure the moisture content quickly and accurately for vegetable seed indnstry. The characteristics and relationships between impedance-frequency and impedance-moisture of the vegetable seeds were studied. The electric conductivity bathtub effect was found out throughout the radio frequency range. The paper presented a new method and designed a new instrument to measure moisture content of vegetable seeds. The experimental results showed that the measuring range of the instrument is 5 %～20 %, the accuracy is higher than 0.5 %, and the uniformity is better than 0.2 %. It can meet the demands for measuring moisture content of vegetable seeds.