In laser drilling of sheet materials, the minimum diameter of the hole processed is determined by the laser machine used. Then, the purpose of this study is to develop a system for minimizing the diameter of the hole and evaluate the performance of the system. A CO2 laser machine, maximum output power of 500 W in single mode, was used in the experiment. This beam includes an area with low power density at the peripheral portion hindering the drilling of a smaller diameter hole. To cope with this problem, an aperture was applied to the system to remove the area described. Metal sheets (0.5 mm thick mild steel and 0.6 mm thick stainless steel) and a ceramic sheet (0.6 mm thick Al2O3) were used as the materials to be processed. The effect of the aperture, together with the effect of pulse conditions such as peak power, frequency and duty, on the diameter and shape of the drilled hole was discussed. Results showed that the diameters of drilled holes are reduced as much as 50%(for mild steel) and 30%(for Al2O3) by the usage of the aperture. It was also found that the drilling of a taper-controlled micro hole is possible by changing the diameter of the aperture used.