  We analyze the semileptonic $D_{q}\to K_1 \ell\nu$ transition with $q=u, d,
s$, in the framework of the three--point QCD sum rules and the nonleptonic
$D\to K_1 \pi$ decay within the QCD factorization approach. We study $D_{q}$ to
$K_1(1270)$ and $K_1(1400)$ transition form factors by separating the mixture
of the $K_1(1270)$ and $K_1(1400)$ states. Using the transition form factors of
the $D\to K_1 $, we analyze the nonleptonic $D\to K_1 \pi$ decay. We also
present the decay amplitude and decay width of these decays in terms of the
transition form factors. The branching ratios of these channel modes are also
calculated at different values of the mixing angle $\theta_{K_1}$ and compared
with the existing experimental data for the nonleptonic case.

High Energy Physics - Phenomenology