The basic and dynamic mechanical properties and thrermal conductivity of rubber composites filled with carbon nanotubes (CNTs) and various particle-sized carbon blacks (CB) were investigated. Results indicated that tear strength and modulus at a definite elongation of rubber composites filled with CNTs were enhanced compared to the conventional CB filler. However, tensile strength and elongation at break became lower. Thermal conductivity of rubber composites partly filled with CNTs is higher than those filled with CB. Rubber filled with the combination of CNTs and the bigger CB particles was beneficial to improve wet-resistant performance and roll resistance of elastomer such as tire.