  We study semileptonic decays $B^\pm \to \eta^{(\prime)} l \nu$, which are
suggested to be used to extract the hadronic form factors of $B$ meson decays
to $\eta(\eta^{\prime})$ and the angle of $\eta-\eta^{\prime}$ mixing. This
would be of great benefit to theoretical studies of $B$ nonleptonic decays
involving $\eta$ and $\eta^{\prime}$, and could lead to a reliable and
complementary determination of $V_{ub}$. The branching ratios are estimated to
be ${\cal B}(B^\pm \to\eta^{(\prime)} l \nu)=4.32 \pm 0.83 (2.10 \pm 0.40)
\times 10^{-5}$, which could be extensively studied experimentally at $BaBar$
and Belle.
