Abstract: Neutrino oscillation is the only phenomenon currently known beyond the standard model of particle physics. It proves that the neutrino mass is not null, a conclusion extremely important to particle physics, astrophysics and cosmology. Among the six parameters for describing neutrino oscillations, two are still unknown: the mixing angle θ13 and the CP phase δ. We propose to build an experiment near the Daya Bay nuclear power station to measure the mixing angle sin22θ13 to a precision of 0.01 at 90% confidence level, an improvement of about one order of magnitude over past experiments. This will provide a guideline for the future development of neutrino physics, and in particular for the understanding of the absence of anti－matter in the universe.