The metastable zone width (MSZW) defines an operating boundary for a crystallization process; thus, it is important to determine the MSZW for a given crystallization process. The application of a simple optical probe consisting of a line laser source, a light-dependent resistor, and a simple processing circuit in the determination of the MSZW of an anti-solvent crystallization process is presented. The optical probe detects the point of nucleation as a sharp change in output voltage upon the onset of nucleation. The isothermal crystallization of glycine from a glycine/water solution using ethanol as anti-solvent was used here to assess the performance of the proposed measurement system. The effects of various operating parameters on the MSZW of glycine were also investigated. The kinetic parameters of nucleation were estimated from the data obtained.