A new method has been developed for retrieving Aerosol Size Distribution (ASD) from spectral aerosol optical thickness (AOT) measured by Microtops II sunphotometer. The rationale of this method is based on a Spectral Extinction Fitting Technique (SEFT) between measured and model-simulated aerosol extinctions. Numerical simulation of aerosol extinctions includes seven aerosol microphysical properties and the resulting total of 64,324,260 datasets were used to compare with Microtops II sunphotometer AOT measurements at five wavelengths (340, 500, 675, 870, and 1020 nm). This SEFT method works directly on AOT measurements and does not require any specific inputs of aerosol properties. The accuracy was examined using AERONET inversion products, and the results show a difference between SEFT and AERONET inversion products of bimodal lognormal volume size concentrations, of ± 10%. The SEFT algorithm was applied for two specific conditions (hazy and clear atmospheres) in Hong Kong, and the corresponding ASD retrievals show reasonable agreements with AERONET inversion products.