In order to clarify the effect of TiB2 particle size on the arc erosion behavior of Ag-TiB2 composite, Ag-4wt%TiB2 composite reinforced by different grain-sized TiB2 particles was prepared by mechanical milling and powder metallurgy. The microstructures of Ag-4wt%TiB2 composites were analyzed, and the global distribution of TiB2 particles was quantitatively evaluated by the enumeration method. The vacuum arc erosion of Ag-4wt%TiB2 composite was tested in a modified TDR240A single crystal furnace. The surface morphology of the Ag-4wt%TiB2 composite after arc erosion was characterized. The mass losses before and after arc erosion were measured. The arc duration was determined by the discharged waveform recorded by a Tektronix TDS-2014 dual channel digital memory oscilloscope, and the arc erosion mechanism was discussed as well. The results show that with the decrease of TiB2 particle size, the TiB2 particles are more uniformly dispersed in the Ag matrix, and Ag-4wt%TiB2 composite presents less mass loss, shorter arc duration, larger arc erosion area and shallower erosion pits. It suggests that fine TiB2 particle can effectively improve the arc erosion resistance of the Ag-4wt%TiB2 composite.