Rosin-grafted polydimethylsiloxane (RGSO) was prepared via ring-opening reaction of glycidyl ester of rosin acid (ER) with hydroxy-terminated amino polydimethylsiloxane (PDMS). The structure of RGSO was confirmed by ¹H and ¹³C NMR spectroscopy. The effects of ER on relative molecular weight and rheological properties of RGSO were studied by gel permeation chromatography and rotational rheometry. Then, room temperature vulcanized (RTV) silicone rubber modified with rosin was prepared using RGSO, hydroxy-terminated PDMS, tetraethoxysilane, and organotin catalyst. The structures and properties of RTV silicone rubbers were studied by scanning electron microscopy, thermogravimetric analysis, a universal testing machine and dynamic mechanical analysis. The rosin-modified silicone rubber showed remarkably improved thermal and mechanical properties. Temperatures corresponding to 10% weight loss and maximum rate of weight loss increased by 66 째C and 177 째C, respectively. Moreover, the tensile strength and elongation at break increased by 138% and 113%. The role of rosin structure in improvement of properties is discussed.