  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.
