Microwave heating is considered to be a potential, efficient and fast method for thermal treatment of biomass. Therefore, elemental understanding of dielectric properties of biomass is necessary before irradiating it with microwave radiations. Dielectric properties along with penetration depth, relaxation time and static permittivity of oil palm shell (OPS) and OPS char were investigated in the frequency ranging between 0.5 and 20 GHz at room temperature. At low frequencies, dielectric constant of OPS was higher than that of OPS char while at higher frequencies, vice versa is observed. Penetration depth for OPS char was higher than that of OPS at low frequencies. Significant variation in relaxation time was observed for OPS in different frequency range, suggesting that the OPS do not obey the Debye equation for dielectrics. Relaxation time and static permittivity for OPS was higher as compared to OPS char. © 2015 Elsevier B.V. All rights reserved.