Three kinds of red clay were fractionated according to the particle size using ultrasonic and centrifugal method. The mechanical properties of the clay-rubber composite with various clay fractions were determined. At the same time, both the content of the metal components in the clay and the affect to the aging properties were determined. The result shows that within a certain range, the clay particle size only affects some of the mechanical properties of clay-rubber composite. The characteristic morphological structure of the clay provides a significant reinforcing action on clay-rubber composite. We speculate that mechanism of the reinforcement of clay on rubber is that the clay forms a rigid or semi-rigid chain aggregation structure in rubber. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 3418-3422, 2009