  We present a study for charmonium radiative transitions:
$J/\psi\rightarrow\eta_c\gamma$, $\chi_{c0}\rightarrow J/\Psi\gamma$ and
$h_c\rightarrow\eta_c\gamma$ using $N_f=2$ twisted mass lattice QCD gauge
configurations. The single-quark vector form factors for $\eta_c$ and
$\chi_{c0}$ are also determined. The simulation is performed at a lattice
spacing of $a= 0.06666$ fm and the lattice size is $32^3\times 64$. After
extrapolation of lattice data at nonzero $Q^2$ to 0, we compare our results
with previous quenched lattice results and the available experimental values.
