We present a calculation of the B and D meson decay constants in lattice QCD with two (Formula presented) flavors of light dynamical quarks, using an O(a)-improved Wilson action for both light and heavy quarks and a renormalization-group improved gauge action. Simulations are made at three values of lattice spacing (Formula presented) fm and four values of sea quark mass in the range (Formula presented) Our provisional estimate for the continuum values of the decay constants are (Formula presented) MeV, (Formula presented) MeV, (Formula presented) MeV, (Formula presented) MeV for (Formula presented) where the statistical and systematic errors are separately listed, and the third error for (Formula presented) and (Formula presented) shows the uncertainty of the determination of the strange quark mass. We also carry out a set of quenched simulations using the same action to make a direct examination of sea quark effects. Taking the ratio of results for (Formula presented) and (Formula presented) we obtain (Formula presented) They show a 10–15 % increase in the (Formula presented) results over those of (Formula presented) for the B meson decay constants, while evidence for such a trend is statistically less clear for the D meson decay constants. © 2001 The American Physical Society.