MnWO4 nanoplates with ionic conductivity were synthesized under hydrothermal condition. In particular, the ionic conductivity of MnWO4 nanoplates is based on the ionic conduction and hygroscopic character in moist atmosphere. When the water molecules are adsorbed, the conductance of MnWO4 will change. At room temperature, the electric capability of MnWO4 is very good (the conductance is 1.46 x 10(-5) S cm(-1)). After being heated for 4 h at 100 degrees C. the conductance of MnWO4 becomes so small that it is on the verge of nonconductor. The water molecules play an important role in the electric conduction of MnWO4. Based on this character, MnWO4 can be used as humidity sensing material, and its possible humidity sensing mechanism is investigated in this paper. (c) 2007 Elsevier B.V. All rights reserved.