Un-doped ZnO and Mg-doped ZnO thin films (TFs) are synthesized by evaporation technique. The synthesized un-doped ZnO TFs are exposed to evaporated Mg contents for different times. XRD patterns reveal the development of (100), (002), (101) planes related to ZnO phase. The shifting of diffraction planes and shrinking of lattice parameters confirms the doping of Mg contents into ZnO lattice. The crystallite size of (100), (101) and (002) planes lies in the range from (12.43-14.51 nm), (14.28-19.08 nm) and (14.78- 19.19 nm) respectively. SEM analysis indicates that the formation of nano-fibers and nano-chains of rounded nanoparticles. EDX analysis confirms the presence of Zn, O and Mg whose amount is increased with increasing exposure time. The energy band gap (Eg) of un-doped ZnO (⁓ 3.05 eV) is increased to  3.08,  3.19 and  3.29 eV) with increasing exposure time in Mg contents.