  We present a high statistics study of the light hadron spectrum and quark
masses in QCD with two flavors of dynamical quarks. Numerical simulations are
carried out using the plaquette gauge action and the O(a)-improved Wilson quark
action at \beta=5.2, where the lattice spacing is found to be a=0.0887(11)fm
from \rho meson mass, on a 20^3\times 48 lattice. At each of five sea quark
masses corresponding to m_{PS}/m_{V} \simeq 0.8-0.6, we generate 12000
trajectories using the symmetrically preconditioned Hybrid Monte Carlo
algorithm. Finite spatial volume effects are investigated employing 12^3 \times
48, 16^3 \times 48 lattices. We also perform a set of simulations in quenched
QCD with the same lattice actions at a similar lattice spacing to those for the
full QCD runs. In the meson sector we find clear evidence of sea quark effects.
The J parameter increases for lighter sea quark masses, and the full QCD meson
masses are systematically closer to experiment than in quenched QCD. Careful
finite-size studies are made to ascertain that these are not due to finite-size
effects. Evidence of sea quark effects is less clear in the baryon sector due
to larger finite-size effects. We also calculate light quark masses and find
m_{ud}^{MS}(2GeV) =3.223(+0.046/-0.069)MeV and
m_s^{MS}(2GeV)=84.5(+12.0/-1.7)MeV which are about 20% smaller than in quenched
QCD.
