  In this work, we study the radiative leptonic decays of $B^-$ and $D^-$
mesons in the standard model and the two-Higgs-doublet-model type II. The
results are obtained using the factorization procedure, and the contribution of
the order $O(\Lambda _{\rm QCD}\left/m_Q\right.)$ is included. The numerical
results are calculated using the wave-function obtained in relativistic
potential model. As a result, the decay mode $B\to \gamma \tau \nu_{\tau}$ is
found to be sensitive to the effect of the charged Higgs boson. Using the
constraint on the free parameters of the two Higgs doublet model given in
previous works, we find the contribution of the charged Higgs boson in the
decay mode $B\to \gamma \tau \nu_{\tau}$ can be as large as about $13\%$.
