We present the measurement of R=B(t→Wb)/B(t→Wq) in pp¯ collisions at s=1.96TeV, using 230 pb−1 of data collected by the DØ experiment at the Fermilab Tevatron Collider. We fit simultaneously R and the number (Ntt¯) of selected top quark pairs (tt¯), to the number of identified b-quark jets in events with one electron or one muon, three or more jets, and high transverse energy imbalance. To improve sensitivity, kinematical properties of events with no identified b-quark jets are included in the fit. We measure R=1.03−0.17+0.19(stat+syst), in good agreement with the standard model. We set lower limits of R>0.61 and |Vtb|>0.78 at 95% confidence level.