Fracture behavior of rubber-modified unsaturated polyester is studied. Unsaturated polyester was modified by the addition of different rubbers; liquid rubbers (BFGoodrich Hycar) and high molecular weight rubbers. Values of stress-intensity factor K-lc and fracture energy G(lc) were determined using a double-torsion test. Youngs modulus was determined in flexural. Increases in toughness were recorded when there was adhesion in the particle-matrix interface. The toughening mechanism was identified. Whitening was not observed. Crack propagation changed from stick-slip to continuous manner when volume fraction of Hycar VTBN increased.