A Monte Carlo simulation study for the two-dimensional antiferromagnetic s=1/2 quantum Heisenberg model is performed using an efficient cluster algorithm developed by present authors. Comparing the finite temperature and finite size effects of various quantities with recent results from the chiral perturbation theory we determine the low energy parameters of the system very precisely. We find that e0= -0. 6693(1)J/a2 for the ground state energy density, Ms= 0.3076(4)/a2 for the staggered magnetization, ?c = 1.68(l)Ja for the spin-wave velocity and ρs= 0.185(5)J for the spin stiffness. Our results agree with the experimental data for the undoped precursor insulators of La2CuO4.