Dielectric properties of bitumen with different thicknesses were investigated by dielectric spectroscopy in the frequency range from 0.1 Hz to 1 MHz and temperature range from (-)40 degrees C to 40 degrees C. The polarisation regions were determined for all samples by using the Cole-Cole equation. While one polarisation region was observed in dielectric constant and dielectric loss factor (tan delta), three polarisation regions were observed in ac conductivity (). These polarisation mechanisms were identified. The effects of thickness on dielectric properties and glass transition temperature (T-g) of bitumen were studied. Toward higher thicknesses (up to 4 mm) the dielectric constant () increased with increasing thickness. After 5 mm, the dielectric constant became independent of thickness that it had a constant value of about 5.5. T-g was affected by sample thickness and it was observed that glass transition temperature varies in the temperature range from (-)50 degrees C to 5 degrees C. and T-g is thickness dependent until the thickness of 5 mm. It was found that T-g obtained from dielectric properties for the bulk phase is compatible with T-g obtained in structural analysis. So, it can be suggested that dielectric spectroscopy may be an alternative analyzing technic for investigation of rheologic properties of bitumen.