This paper presents a novel method for monitoring the laser-drilling process for a hole by using machine vision. Through on-line image acquisition and analysis during the laser-drilling processes, we could simultaneously obtain correlations between the machining processes and the analyzed images. The depths of laser-machined holes could be estimated on-line by using the machine vision method. Therefore, an inexpensive on-line inspection system was developed to increase productivity. All of the processing work was performed in air under standard atmospheric conditions with gas assist. A correlation between the area of radiation emitted from the vaporized material and the depth of the hole was derived. The results indicate that the estimated depths of the laser-drilled holes were linearly proportional to the thresholded image area, with a high degree of confidence. This research provides a novel online machine vision-based method for estimating the depths of laser-drilled holes.