We investigated the evolution of the magnetic structure and magnetoelastic effects of NdMnO(3) by neutron powder diffraction. We confirmed the A-type antiferromagnetic (AF) structure of NdMnO(3) below T(N)≈82 K with magnetic moments parallel to the b axis of the orthorhombic crystal structure (space group Pbnm). We found that the magnetic moments of Nd order below about T(N)(Nd)≈20 K in a ferromagnetic structure with moments parallel to the c axis. At the same temperature the magnetic moments of Mn develop a ferromagnetic component parallel to the c axis. We found strong magnetoelastic effects associated with the AF transition at T(N)≈82 K. The effect is very prominent for the b lattice parameter. All three lattice parameters and therefore the unit cell volume contracts at T(N)≈82 K. We treated quantitatively the magnetoelastic effect of the b lattice parameter and found that the extra change in the lattice parameter Δb due to the magnetoelastic effect is proportional to the magnetic moment of the Mn ion. We also determined the critical exponent of the AF phase transition to be β = 0.296 ± 0.008.