This paper primarily investigates the possible optimum conditions for maximum yield production of hydrochar through hydrothermal carbonization of palm shell. The hydrochar and the palm shell have been characterized to understand the effect of hydrothermal carbonization on palm shell and the chemical, the dielectric, and the structural properties of optimized hydrochar were examined. The effect of the reaction temperature, reaction time and biomass to water ratio was analyzed and optimized using the central composite design of response surface methodology. The optimized conditions for hydrochar production have been found to be 180 degrees C, 30 min, and 1.60 wt.%, temperature, time and biomass to water ratio, respectively. The findings showed that the temperature has a greater influence over the efficiency of hydrochar production than time and biomass-to-water ratio. The porosity of raw palm shell increased remarkably through hydrothermal carbonization. The dielectric constant of palm shell and hydrochar was decreased with an increase in frequency. (C) 2015 Elsevier Ltd. All rights reserved.