The optical and electronic properties of graphane and silicane are studied by first-principles method based on density functional theory, employing both the hybrid (HSE06) and the Perdew-Burke-Ernzerhof (PBE) functionals. Our HSE06 results show that graphane has a direct band gap of 4.49 eV, while silicane has an indirect band gap of 2.94 eV. In graphane, the imaginary part of dielectric function has two peaks at 7.70 and 15.03 eV for E||z, respectively, while for E||x, there are three peaks lying at 11.56, 13.25 and 14.33 eV, respectively. In silicane, has a main peak at 8.68 eV for E||z, while there are two peaks at 4.56 and 8.55 eV for E||x, respectively. In graphane, the peaks in are mainly due to the transitions from C 2p and H 1 s states to C 2 s and 2p ones; while in silicane, the peaks are primarily originated from the transitions from Si 3p and H 1 s states to Si 3 s and 3p as well as H 1 s states.