  We explore sea quark effects in the light hadron mass spectrum in a
simulation of two-flavor QCD using the nonperturbatively O(a)-improved Wilson
fermion action. In order to identify finite-size effects, light meson masses
are measured on 12^3x48, 16^3x48 and 20^3x48 lattices with a~0.1 fm. On the
largest lattice, where the finite-size effect is negligible, we find a
significant increase of the strange vector meson mass compared to the quenched
approximation. We also investigate the quark mass dependence of pseudoscalar
meson masses and decay constants and test the consistency with (partially
quenched) chiral perturbation theory.
