Abstract: An efficient sensing method based on a flow injection manifold has been established for spectrophotometric determination of metoclopramide. The method is based on simple diazotization reaction of metoclopramide with nitrite in acidic medium prior to azo coupling reaction with graphene quantum dots (GQDs). Absorbance was monitored at 423 nm. Effects of various chemical and instrumental parameters were investigated. The analysis of the results of central composite design showed that for obtaining the maximum sensitivity, concentrations of nitrite and hydrochloric acid, and dilution factor for GQDs should be as 1.31 mmol L-1, 0.39 mol L-1 and 2-fold dilution, respectively. Two calibration linear ranges were observed for the determination method. The ranges were: 0.1-10 and 10-40 μg mL-1 of metoclopramide. The method was successfully applied to the determination of metoclopramide in pharmaceutical tablets, without any interference from common excipients. Also, the method was successful for quantitation of metoclopramide in different water matrices.