The electronic and optical properties of the pristine and functionalized adamantane molecules were investigated using the time-dependent density functional theory (TD-DFT). Tertiary hydrogen atoms of adamantane were replaced with lithium (Li) and sodium (Na) atoms to form the C10H16-NXN structure (X = Li or Na, N = 1, 2, 3, 4). We found that the pristine adamantane has a large band gap and negative electron affinity. In contrast, all of the functionalized adamantane molecules shown that adding more Li and Na atoms leads to reduction of the energy gap and increasing of the positive electron affinity. Furthermore, the calculated optical spectra of the functionalized adamantane were found in the infrared region. This feature characteristic suggests that the adamantane functionalized with Li or Na atoms is potential application for optoelectronic device in the infrared region