Fluorescence decay curves of Rh6G with different concentrations of KI in water were measured by using a photomultipler tube and a sampling oscilloscope. The response time of the system was in the order of nanoseconds. The observed fluorescence decay curves were convolutional results of the real decay curves and the instrumental response curve. A combination of the phase plane method and iterative convolution method was used in making deconvolution to obtain the real fluorescence lifetimes. The minimum time resolution was found to be 130 picoseconds, that is, 3% of the FWHM of the instrumental response time.