  We present a measurement of the Cabibbo-Kobayashi-Maskawa matrix element
|Vcb| based on a sample of about 53,700 B0bar --> D*+ l- nubar_l decays
observed by the BaBar detector. We obtain the branching fraction averaged over
l = e, mu,
  BR(B0bar --> D*+ l- nubar_l) = (4.90 +- 0.07(stat.) +0.36-0.35(syst.))%.
  We measure the differential decay rate as a function of w, the relativistic
boost gamma of the D*+ in the B0bar rest frame. By extrapolating dGamma/dw to
the kinematic limit w --> 1, we extract the product of |Vcb| and the axial form
factor A_1(w=1). We combine this measurement with a lattice QCD calculation of
A_1(w=1) to determine |Vcb| = (38.7 +- 0.3(stat.) +- 1.7(syst.)
+1.5-1.3(theory)) * 10^-3.
