Determination of soil water content using the Time Domain Reflectometry (TDR) method is recently considered modern and widely used. The practical application of TDR requires its calibration ie determination of the relationship between the dielectric constant and the volumetric moisture content of the soil. The paper presents the development of calibration equations for the range of different peat types (willow peat, sedge-moss, reed and sedge-reed) from the Biebrza river valley. Undisturbed soil samples were used in the calibration procedure. The volumetric moisture content and the dielectric constant were measured simultaneously during the calibration. The study showed that bulk density substantially affects the relation between the dielectric constant and moisture content in peat soils. The empirical calibration equation accounting for bulk density values ranging from 0.08 to 0.25 g cm-3 was found.