Optical single backscattering spectroscopy can be used for sizing spherical particles suspended in water, but the accuracy for sizing submicron particles cannot be satisfied. To improve the accuracy, the backscattering coefficients between 300~400 nm spectral region is applied to the determination of diameter of calibrated spherical latex polystyrene particles. Through Mie calculations, fast Fourier transform (FFT) and linear regression, the diameters of particles covering the range between 0.25 μm and 1 μm for monomodal particles are determined. The linear regression average error is ±0.02 μm, which is reduced in more than an order of magnitude compared to the data given by the preference. The method can also be applied to bimodal samples.