We report on our study of light hadron spectrum and quark masses in QCD with two flavors of dynamical quarks. Simulations are made with the plaquette gauge action and the non-pertubatively O( a) improved Wilson quark action. We simulate 5 sea qaurk masses corresponding to m PS/ m V ⋍ 0.8-0.6 at β = 5.2 on 12 3 × 48, 16 3 × 48 and 20 3 × 48 lattices. A comparison with previous calculations in quenched QCD indicates sea quark effects in meson and quark masses.