We report an on-line Raman spectroscopy measurement with fiber probe. In the experiment, the real-time spectrum measurement of ethyl acetate synthesis reaction with ethanol and acetic acid was carried out by using a homemade Raman spectrometer. The time variation of integrated spectrum peak intensities of reaction product and raw materials are obtained. From the integrated peak intensities, the concentrations of the reactants can be deduced and the ending point of reaction under this experimental condition can be determined.