The influence of the number of detectors is observed by comparing a given bimodal particle size distribution (PSD) and a calculated bimodal PSD by computer simulation and experimental method. A nonlinear iteration based on the Twomey method is used for calculating PSD. An observed range of scattering angles is 5.0x10(-4) to 2.5rad, and the Mie light scattering theory is used. The number of detectors was changed from 6 to 81 elements by the computer simulations. The algorithm of the inversion problem is applied with mean diameters of lognormal distribution in a range from 0.546 to 422 mu m at a standard deviation of 0.01 and 0.27. Experimental bimodal samples are prepared by certified monodisperse polystyrene latex standards, and results are obtained by three different kinds of optics, which has 21, 41, and 81 elements detector, respectively. The polydisperse bimodal PSD converges from 42 elements of the number of detectors. The bimodal PSD of monodisperses or size range of PSD under 50 mu m converges from 21 elements of the number of detectors. The accuracy of calculated PSD is effected not only by the number of detectors but also the shape of the scattering function.