Quasi-elastic light scattering is utilized to measure the mean diameter of the 0.3 $mu extrm{m}$-dimeter polystyrene sphere. The mean diameter of the polystyrene sphere is required to be known within the uncertainty of a few per centages. The systematic error has been considerably reduced by improving the system for the angle alignment and temperature measurement of cell. NIST SRM 1691 (0.269$pm$0.007 $mu extrm{m}$: TEM; 0.275$pm$0.007 $mu extrm{m}$: QELS) is measured to be 0.273 $mu extrm{m}$$pm$0.006 $mu extrm{m}$ in mean diameter. Detailed description is given of the improved method and the resultant uncertainty, and the comparison of results with NIST is followed.