The Z-scan technique is a simple and popular experimental technique to measure intensity dependent nonlinear susceptibilities of materials; way to obtain information of nonlinear refractive index and nonlinear absorption properties of materials. The schematic of the automised Z-scan setup is shown in figure 1. The materials showing nonlinear absorption found application in fabrication of optical limiting devices, that means materials whose transmission falls with increasing incident intensity. ZnO nanoparticles were prepared at room temperature by wet chemical route without any capping agents using 0.1 mol of zinc acetate dehydrates and 0.075M NaOH in methanol and stirred for 2 hours and these nanoparticles were embedded in PMMA matrix. Figure 2 shows the open and closed aperture Z-scan traces of the ZnO/PMMA nanocomposites.