We present results on an analysis of the decay constants fBd and fBs with two flavours of sea quark. The calculation has been carried out on 3 different bare gauge couplings and 4 sea quark masses at each gauge coupling, with mπ/m ranging from 0.8 to 0.6. We employ the Fermilab formalism to perform calculations with heavy quarks whose mass is in the range of the b-quark. A detailed comparison with a quenched calculation using the same action is made to elucidate the effects due to the sea quarks.