The size change of silver agglomerate particles with diameters less than or equal to 100 nm during sintering is investigated. The growth rate of primary particles due to partial coalescence within an agglomerate is first derived based on a sintering theory. In the experiment, silver agglomerates uniform in electrical mobility-equivalent diameter are introduced into a heating pipe with N-2 gas flow. The size changes of the agglomerates are measured using an aerosol technique. Agglomerates heated up to about 500 K (about 0.4 of the melting point) decrease steeply in size with temperature. The agglomerates which have fused into single particles begin to evaporate at a temperature > 900 K. The measured change in diameter with temperature agree well with theoretical calculations in which suppression of sintering due to primary particle growth is taken into account.