This study aimed to develop an automated optical inspection (AOI) system that can rapidly and precisely measure the dimensions of microchannels embedded inside a transparent polymeric substrate, and can eventually be used on the production line of a factory. The AOI system is constructed based on Snell’s law. The concept holds that, when light travels through two transparent media (air and the microfluidic chip transparent material), by capturing the parallel refracted light from a light source that went through the microchannel using a camera with a tele-centric lens, the image can be analyzed using formulas derived from Snell’s law to measure the dimensions of the microchannel cross‐section. Through the NI LabVIEW 2018 SP1 programming interface, we programmed this system to automatically analyze the captured image and acquire all the needed data. The system then processes these data using custom‐developed formulas to calculate the height and width measurements of the microchannel cross‐sections and presents the results on the human–machine interface (HMI). In this study, a single and straight microchannel with a cross‐sectional area of 300 μm × 300 μm and length of 44 mm was micromachined and sealed with another polymeric substrate by a solvent bonding method for experimentations. With this system, 45 cross‐sectional areas along the straight microchannel were measured within 20 s, and experiment results showed that the average measured error was less than 2%.