Al doped ZnO nanoparticles (Zn_1−xAl_xO, 0.00$\le x\le 0.05$) have been synthesized by the co-precipitation method at temperature of 500 °C for 2 h in air. The synthesized samples were characterized by powder X-ray diffraction (XRD), FE-SEM/EDS, TEM and UV–visible spectrophotometer. The prepared nanoparticles exhibit a hexagonal wurtzite structure with an average particle size of 26 nm as evident from XRD measurements. The calculated average crystalline size of particle varied from 20 to 39 nm with increase in Al concentrations. The increase in lattice parameters reveals the easy fit of Al into ZnO lattice. It was observed that the Al doped ZnO nanoparticles (Zn_1−xAl_xO, 0.00$\le x\le 0.05$) exhibit higher transmittance i.e. above 77% at 400 nm wavelength as compared to the un-doped ZnO nanoparticles (70%) due to its smaller crystal size (23.9 nm) as compared to un-doped ZnO nanoparticles (77.4 nm). Also, Al doped ZnO nanoparticles showed higher optical energy band gap as compared to un-doped ZnO nanoparticles.