Applications of microwave techniques in the wood industry have been developed both for treatment and for diagnostic purposes. All these applications require reliable results of the dielectric constant of the wood species of interest. In this paper, the dielectric constant of the most widely used tree species growing in Turkey were determined at 9.8 GHz in the longitudinal direction for moisture conditions from ovendry to 28 percent at a room temperature of 22degrees to 24degreesC. Results indicated that the behavior of all wood species studied is quantitatively similar. In general, the dielectric constant increases within the range studied with increasing moisture content. The differences in dielectric behavior among wood species are related to their specific characteristics . There exists a positive linear relationship between specific gravity and the dielectric constant.