Cosmic strings had been studied originally as potential seeds of the large-scale structure of the Universe, which had eventually been replaced by inflation, but the recent concern on them would be an aspect as a possible source of gravitational waves. The cosmic strings have a network structure spread over superhorizon scales, and their reconnection process is crucial for their networks to evade the strong observational constraints and thus to survive in the present time. In this talk, we show our recent numerical works on the reconnection process and the time-evolution of cosmic strings networks based on the 3D field-theoretic simulations. Furthermore, we introduce some preliminary results on the gravitational waves radiated from the cosmic strings networks.