We present an exploratory lattice study for the two-photon decay of $$\eta _c$$ η c using $$N_f=2$$ N f = 2 twisted mass lattice QCD gauge configurations generated by the European Twisted Mass Collaboration. Two different lattice spacings of $$a=0.067$$ a = 0.067 fm and $$a=0.085$$ a = 0.085 fm are used in the study, both of which are of physical size of 2 fm. The decay widths are found to be 1.025(5) KeV for the coarser lattice and 1.062(5) KeV for the finer lattice, respectively, where the errors are purely statistical. A naive extrapolation toward the continuum limit yields $$\Gamma \simeq 1.122(14)$$ Γ ≃ 1.122 ( 14 ) KeV, which is smaller than the previous quenched result and most of the current experimental results. Possible reasons are discussed.