  In this paper, we studied the semileptonic decays $B \to D^{(*)} l^-
\bar{\nu}_l$ by using the "pQCD+Lattice QCD" method. We made the extrapolation
for the six relevant form factors by using the input values obtained from the
pQCD factorization approach in the low $q^2$ region of $0\leq q^2 \leq
m_\tau^2$, and the Lattice QCD input at the end-point $q^2=q^2_{\rm max}$. We
then calculated the ratios $R(D)$ and $R(D^*)$ of the branching ratios ${\cal
B}(B \to D^{(*)} l^- \bar{\nu}_l)$, and found numerically that (1) the
"pQCD+Lattice QCD" predictions for the branching ratios ${\cal B}(B \to D^{(*)}
l^-\bar{\nu}_l)$ agree well with the measured values within one standard
deviation, and (2) the "pQCD+Lattice QCD" predictions for the ratios
$R(D^{(*)})$ are $ R(D)=0.337^{+0.038}_{-0.037}$ and $
R(D^*)=0.269^{+0.021}_{-0.020}$, they agree with the data within $2\sigma$
deviation, in other words, one can explain the "$R(D^{(*)})$-puzzle" in the
framework of the standard model.
