Pure and Sn-doped ZnO nanostructures have been synthesized by the microwave irradiation method. The influence of Sn loading on the morphology and microstructure was evaluated by using field emission scanning electron microscopy, transmission electron microscopy (TEM), energy-dispersive spectrum analysis techniques, X-ray diffraction, and Fourier transform infrared spectroscopy. A change in the growth pattern, from needle-like particles for pure ZnO to agglomerated spherical crystallites for Sn-doped ZnO, has been observed. TEM observations indicated that the average particle size of the pure ZnO nano needles is in the range of 40-60 nm, whereas on addition of Sn spherical nanoassemblies size lies in the range of 10-21 nm. The pure ZnO and Sn-doped ZnO nanostructures were further characterized for their optical properties by UV-Vis reflectance spectra (DRS) and photoluminescence (PL) spectroscopy.