We report an ion/neutral reaction of sympathetically laser-cooled molecular ions. Using laser-cooled fluorescence mass spectrometry, a reaction of about 100 sympathetically cooled H3O+ ions with NH3 were traced at 4 min interval as the reacting ions were kept within a radiofrequency-quadrupole ion trap. To obtain the reaction rate of the reaction molecular dynamical calculation was used to relate the fluorescence mass spectra to the quantity of the molecular ions.