Mg(x)Zn1-xO nanoneedles were synthesized on alumina substrate by using thermal oxidation technique under normal atmosphere. Zn powder and MgO powder were mixed and heated to form Mg(x)Zn1-xO with x content of 0-0.3 by mol at heating temperature and time of 400-1000 degrees C and 24 h. The morphology of Mg(x)Zn1-xO nanoneedle was characterized by filed emission scanning electron microscope (FE-SEM). It was found that the needle-like nanostructures with the sharp ends were observed outward from microparticles at 400-800 degrees C. From EDS, XRD, and TEM analysis, it was suggested that Mg(x)Zn1-xO alloy was formed with no segregation of MgO in Mg(x)Zn1-xO alloy after thermal oxidation process. Also, from reflectance spectra, the Mg(x)Zn1-xO nanoneedle exhibited higher energy gap than that of ZnO films for entire Mg content indicating widening band gap energy due to alloying effect. Moreover, we have proposed the growth mechanism of Mg(x)Zn1-xO nanoneedles based on growth kinetic of nucleation formation. This growth model can be explored to explain nanostructure of other metal-oxide alloy prepared by thermal oxidation.