An unsaturated polyester resin is prepared by the ester interchange between p-carbethoxy phthalanilic acid and maleic anhydride with ethylene glycol in the ratio of (1:1:2.1 mel). The structure of this polyester was established by studying its IR and NMR spectra and the acid number was determined. The effect of 5 phr polyester on the mechanical and physical properties of two types of rubber namely styrene-butadiene rubber (SBR) and acrylonitrile-butadiene rubber (NBR) was studied. The ultrasonic velocity and attenuation have been investigated for both types of rubber with and without the polyester resin at 2 and 5 MHz in the temperature range between 180 and 346 K. For each type of rubber two relaxation peaks are observed in these temperature and frequency ranges, a main peak and a secondary one. The addition of the polyester resin is found to affect the position and height of the main peak in SBR but not in NBR. It was found that the apparent activation energy of the main relaxation process is temperature dependent and increases with the addition of the polyester resin in both types of rubber.