  The mixing of $\eta-\eta'$ or $\eta-\eta'-G$ is of a great theoretical
interest, because it concerns many aspects of the underlying dynamics and
hadronic structure of pseudoscalar mesons and glueball. Determining the mixing
parameters by fitting data is by no means trivial. In order to extract the
mixing parameters from the available processes where hadrons are involved,
theoretical evaluation of hadronic matrix elements is necessary. Therefore
model-dependence is somehow unavoidable. In fact, it is impossible to extract
the mixing angle from a unique experiment because the model parameters must be
obtained by fitting other experiments. Recently $BR(D\to\eta+\bar l+\nu_l)$ and
$BR(D_s\to\eta(\eta')+\bar l+\nu_l)$ have been measured, thus we are able to
determine the $\eta-\eta'$ mixing solely from the semileptonic decays of
D-mesons where contamination from the final state interactions is absent. Thus
we hope that the model-dependence of the extraction can be somehow alleviated.
Once $BR(D\to\eta'+\bar l+\nu_l)$ is measured, we can further determine all the
mixing parameters for $\eta-\eta'-G$. As more data are accumulated, the
determination will be more accurate. In this work, we obtain the transition
matrix elements of $D_{(s)}\to \eta^{(\prime)}$ using the light-front quark
model whose feasibility and reasonability for such processes have been tested.
