The laser-induced fluorescence technique has been used to measure primary biological aerosol particles (PBAPs) in the atmosphere because of its ability to classify the origins of particles based on the combination of excitation and detection wavelengths. However, information on the fluorescence intensity related to the amounts of fluorescent compounds in airborne particles measured by this method is limited. Therefore, in this study, the scattered and fluorescent signals from fluorescent polystyrene latex (PSL) particles were observed simultaneously through laser-induced fluorescence. The ratio of the fluorescence to scattered signal intensities from individual particles was found to be almost equal for particles of the same size. Consequently, mixed-size fluorescent particles can be observed separately by simultaneously measuring the fluorescence and scattered signals from individual particles. Mie theory was used to calculate relative scattered signal intensities. Incorporating ratios of fluorescence to scattered signal and relative scattered signal intensities, relative fluorescence intensities were obtained. These results suggest that the relative fluorescence intensities are almost linear with respect to the square of the particle size.