In microelectronics processing, coating of photoresist is a common process. It is important to ensure the uniformity of the photoresist thickness across the wafer. In this paper, we propose an in-situ monitoring system. In the setup, a spectrometer is used to measure the photoresist thickness contour on the wafer after the spin-coat step or edge-bead removal step. The experimental results are compared with off-line ellipsometer measurements. The worst-case error is experimentally found to be less than 2%.