  In this work, we study the radiative leptonic decays of $B^-$ and $D^-$
mesons using factorization approach. Factorization is proved to be valid
explicitly at 1-loop level at any order of $O(\Lambda_{\rm
QCD}\left/m_Q\right.)$. We consider the contribution in the soft photon region
that $E_{\gamma} \sim \left. \Lambda^2_{\rm QCD} /\right. m_Q$. The numerical
results shows that, the soft photon region is very important for both the $B$
and $D$ mesons. The branching ratios of $B\to \gamma e\nu_e$ is $5.21\times
10^{-6}$, which is about $3$ times of the result obtained by only considering
the hard photon region $E_{\gamma}\sim m_Q$. And for the case of $D\to \gamma
e\nu_e$, the result of the branching ratio is $1.92\times 10^{-5}$.
