Pumice stone (P), which is as a cheap material, can be a good candidate for filling material for polymers because of its high thermal resistance. Its effect on properties of polypropylene (PP) has not been studied in detail up to now. The mechanical, viscoelastic, thermal, morphological, crystallographic, and thermomechanical behaviors of PP filled with P powder were studied. The composites were produced by adding different weight fractions of P powder (0–40 wt%) into PP using a high-speed thermo kinetic mixer and compression molding. The highest tensile and flexural strength values, 26.5 and 46.4 MPa, respectively, were obtained when 10 wt% of P powder was filled into PP. Besides, the storage and loss modulus of PP was increased with the addition of P powder. The onset decomposition temperature of PP increased by 39°C with P powder loading into PP. The addition of P powder into PP led to a significant increase on the crystallinity of PP. It is interesting to report that thermal conductivity of PP was increased with increasing weight fraction of P powder.