We employed very long nanowires mixed into nanoparticle thin films to improve their weak crack resistance for flexible electronics. We also measured the dependence of the failure strain and resistivity of the Ag nanowire-reinforced Ag nanoparticle thin films on the wt% and sintering condition of nanowires by means of resistance measurements under tension in an in situ tensile tester. Very long nanowires of over 90 μm suppressed thin film cracking by bridging cracks in both the transverse and longitudinal directions and also reduced the electrical resistivity under tension. Moreover, diffusion and growth of nanowires induced by the thermal annealing process was observed, resulting in a highly increased failure strain. Due to the use of the same materials, effective diffusion was facilitated, thus inducing strong bonds between nanoparticles and nanowires.