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-perturbatively $O(a)$ improved Wilson quark action. We simulate 5 sea qaurk masses corresponding to $m_{PS}/m_{V} simeq 0.8$--0.6 at $beta=5.2$ on $12^3 times 48$, $16^3 times 48$ and $20^3 times 48$ lattices. A comparison with previous calculations in quenched QCD indicates sea quark effects in meson and quark masses.