The first report on lithium nanoparticles synthesized experimentally is presented. The nanoparticles were obtained using the extract of Opuntia ficus-indica through “green synthesis”. The optical, structural and vibrational properties were studied through optical absorption, TEM microscopy and Raman spectroscopy. The particles show a size of approximately 5nm. The absorption bands were found at 315 and 415nm in the samples after the synthesis of the nanoparticles. One Raman band was detected at around 280cm−1. Several levels of approximation of the density functional theory and the Hartree–Fock method on the structural and vibrational study of the lithium clusters (Li n , n =2–18) are incorporated. A radial breathing mode (RMB) type Raman mode was found for the case Li n (n >3) in the predicted Raman spectrum. This mode shows the highest relative intensity for each case, and a shift to lower wavenumbers when the amount of atoms in the cluster increases.