From a viewpoint of increasing the throughput of particle separation using laser radiation pressure, the long-range movements of a micron-order particle in a weakly focused Gaussian beam are investigated using a three-dimensional analytical model of the particle movement. The concept of the I separation area' is introduced as an indicator of the throughput. The effects of numerical aperture of the focusing lens, laser power and the medium flow velocity on the throughput are examined through the calculation of the separation area of a soda-lime glass micro-sphere moving in water. Then, the potential applications of this particle separation are discussed on the basis of the difference in the separation area for particles with various refractive indices. Results show that the optimum numerical aperture in a static medium is obtainable in a very narrow region above a critical value. Deterioration of particle throughput due to the flow of a medium is controlled by using a numerical aperture slightly larger than the critical value. A high-power laser is advantageous to increase the particle separation throughput. Furthermore, separation of transparent and non-transparent particles can be readily achieved under optimal conditions of the optical system and a medium flow-field, whereas separation of transparent particles requires a large difference in the refractive index and uniformity in size.