Abstract: In order to realize online determination of coal moisture at places, for example in underground coal mines, with explosion-proof requirements, a microwave online moisture meter is specifically developed based on the principle that the power of microwave tends to attenuate in the presence of coal moisture. The meter works in the C-waveband. For the development of the meter, modeling is made based on linear relationship between coal moisture and microwave attenuation and by using the coal quantity compensation method. The overall structure of the meter including the microwave transmitter, antenna, solids level sensor is all made explosion-proof in design for mine use. It is assessed through in-plant testing that the use of the meter can meet the requirements, with a RMS deviation of less than 0.5%. Finally, suggestions on practical use of the meter in mines are put forward.