We present a calculation of the B and D meson decay constants in lattice QCD with two (Nf=2) flavors of light dynamical quarks, using an 0(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 a = 0.22,0.16,0.11 fm and four values of sea quark mass in the range mPS/invO.S- 0.6. Our provisional estimate for the continuum values of the decay constants are fBd = 208(10)(29) MeV, fBs = 250(10)(35)(-0 +8) MeV,fDd=225(14)(40) MeV,fDj=267(13)(48)(°) MeV for Nf=2 where the statistical and systematic errors are separately listed, and the third error for/Bj and/Dj 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 Nf=2 and Af=0, we obtain fbd Nf=2/2T°=1.11(6)£r 2//£f°= U4(5)4r11°= l.03(6)4f 2/£f °= 1.07(5). They show a 10-15% increase in the Nf=2 results over those of Nj=Q 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.