We present numerical simulation of QCD with two dynamical quark flavors described by the overlap fermion action on a $16^3times 32times (0.12 {rm fm})^4$ lattice. We calculate pseudo-scalar masses and decay constants and investigate their chiral properties. We test the consistency of our data with the two-loop chiral perturbation theory predictions, which should also be valid at finite lattice spacings because of the exact chiral symmetry, including the finite size effects.