The kerf width size variation along a laser-cut section lowers the end product quality significantly. In the present study, CO2 laser cutting of mild steel is considered and the influence of laser power and cutting speed variations on the kerf width size is examined. A lump parameter analysis is introduced when predicting the kerf width size and an experiment is conducted to measure the kerf size and its variation during the cutting process. It is found that the power intensity at the workpiece surface influences significantly the kerf width size. The variation in the power intensity results in considerable variation in the kerf size during the cutting, which is more pronounced at lower intensities.