The Cu-10Ag and Cu-1OAg-RE (RE = Ce, Y) alloys in situ filamentary composites were prepared. The relationships of the ultimate tensile strengths ( UTS) and microstructure changes of the composites were studied. With increasing of the true strain η, the sizes of the Ag filaments in the composites reduce according to a negative exponential function of η : d = d0·exp (-0. 228η) , and the UTS of the composites increase also according to a exponential function of η, σcu/Ag = σ0(cu) + [Kcu/Ag d0 -1/2 ]exp( η/3), here d0 is a coefficient related to the original size of Ag phase. The strain strengthening follows a two-stage strengthening effect. The strengthening mechanisms are related to changes of microstructure in the deformation process. At the low true strain stage, the strengthening is mainly caused by the working hardening controlled by dislocation increasing; at the high true strain stage, the strengthening is mainly caused by the super-fine Ag filaments and the large coherent interfaces between the Ag filaments and Cu matrix. The trace RE additions and the rapid solidification obviously refine scales of the Ag filament of the composites, and therefore obviously increased the strain strengthening rate. The microstructure refinement of the composites, especially the refinement of Ag filament, is the main reason of the high strain strengthening effect in Cu-Ag alloy in situ filamentary composites.