Two unsaturated polyesters, one based on phthalic anhydride PE P and the other based on isophthalic acid PE I, were synthesized. The chemical structure of the two polyesters was characterized by IR and 1 H and 13 C NMR spectroscopy. The effect of styrene concentration on the curing of polyesters was also studied. It has been found that the percent of polyester/styrene (70/30 wt %) gave the highest percent of curing. Different concentrations of carborundum (0-70 wt %) were used to prepare polyester composites. A comparative study was done on the properties of the two prepared polyesters PE P and PE I and their composites in term of their thermal, mechanical, electrical, and physical properties. The results indicate that the polyester based on isophthalic acid (PE I ) and its composites gave higher compressive strength values and lower water absorption than those based on phthalic anhydride (PE P ). The presence of carborundum improved the thermal stability than the cured polyesters and electrical properties.