The relation between the microstructure and properties of YBa2Cu3O7-delta-Ag-x composite superconductors prepared via sol-gel processing has been studied. Optical microscopy, SEM and TEM analyses indicated that Ag-doped samples were dense, with small and regular grains and narrow spaces between grains. Ag particles were distributed homogeneously in samples and these could increase their J(0) values. The mechanisms of the effect of Ag doping on the superconducting properties and microstructure of samples were hindrance of the movements of grain boundaries and metallization of grain boundaries. However, too much Ag doping would weaken the coupling among superconducting grains and induce J(0) to decrease. The experimental results confirmed these ideas.