  We study the form factors of the $\eta_c$ meson in the light-front quark
model. We explicitly show that the transition form factor of $\eta_c \to
\gamma^* \gamma$ as a function of the momentum transfer is consistent with the
experimental data by the BaBar collaboration, while the decay constant of
$\eta_c$ is found to be $f_{\eta_{c}}=230.5^{+52.2}_{-61.0}$ and
$303.6^{+115.2}_{-116.4}$ MeV for $\eta_c\sim c\bar{c}$ by using two $\eta_c
\to \gamma \gamma$ decay widths of $5.3\pm 0.5$ and $7.2\pm2.1$ keV, given by
Particle Data Group and Lattice QCD calculation, respectively.
