Undoped and 20at. % Al doped ZnO nanocrystals were successfully prepared by Sol-gel auto combusion route. Structural properties were studied from XRD pattern. The hexagonal wurtzite structure was detected. The crystallite size was calculated using Sherrer’s formula. Crystallite size determined 5.68nm and 4.05nm for undoped and 20at.% Al concentration. Optical study was completed by UV-Visible spectroscopy. Absorption peaks estimated at 352 nm and 366 nm. Energy band gap were calculated from Tauc’s plots. 3 eV and 3.05 eV are the values of Eg for ZnO and 20 at.% Al doped ZnO nanocrystals. It confirms that red shift in the energy band gap.