AbstractKenaf fiber-filled polyurethane foams were prepared using the free rising method. The dielectric constants and the loss tangents of the composites were studied as functions of fiber content (0, 5, 10, and 15 parts per hundred of polyols by weight), temperature (from 30 to 200 °C), and electric field frequency (from 20 Hz to 2 MHz). The dielectric constant and the loss tangent increased with increasing fiber content. The dielectric constant was very high in the range of 101 to 102 Hz and varied little in the range of 103 to 106 Hz, but decreased rapidly above 106 Hz. The loss tangent decreased as the frequency increased. The effect of frequency on the loss tangent value was greater at frequencies below 102 Hz. Higher temperatures led to a higher dielectric constant and loss tangent. When the temperature was above approximately 120 °C, the loss tangent dramatically increased. The incorporation of kenaf fiber can improve the growth rate of the dielectric constant with increasing temperature. The dielectric constant and the loss tangent increased with increasing fiber content, indicating that both the dielectric capability and energy dissipation ability of the composites were improved.