The nanocomposite Ag-SnO2 and La-doped Ag-SnO2 electrical contact alloys are prepared by chemical co-precipitation method and hot pressing and sintering technique. The main physics performances were measured in this paper. The breakdown field strength of contacts alloys were investigated in vacuum and the microstructures are observed by field emission scanning electron microscopy. The results show that the nanosized SnO2 of the same morphology and grain size as the prepared powders，disperses evenly in the Ag based. Especially for the La-doped nanocomposite Ag-SnO2 contact alloys, the oxide grain size is less, and disperses more evenly. With the oxide grain size decreasing, the hardness and electrical resistivity increase, and the breakdown field strength and the arc erosion rate decrease.