Abstract: The study of the quantitative contribution of the thermal volume expansion to the positive temperature coefficient (PTC) switch effect of fine particles filled polymer composites in the carbon black-polyethylene(CB-PE) system is reported. The decrease of room temperature resistivity with the increase of CB loading level (CB volume fractions at room temperature) experimentally obtained was fitted by General Effective Medium (GEM) equation in order to establish the mixing rule of the system. A dilution of the CB volume fraction due to the thermal volume expansion of the composites is estimated and "real" CB volume fractions of a certain loading level at different temperatures are thus found. Being treated equivalent to the loading levels, the "real" CB volume fractions at high temperatures are used to replace the volume fraction in the above GEM equation. By using this model, the contribution of thermal expansion of matrix to the jump-like PTC switch transition of the composites is quantitatively estimated. The theory and experiment have proved that the conductive mechanism of abrupt resistivity increase at PTC transition range is equivalent as abrupt resistivity increase at the percolation curve close to the critical volume fraction.