Fluorescence Correlation Spectroscopy (FCS) is one of the most popular techniques in the studying of intramolecular dynamics as well as molecular interactions of biomolecules in the local microenvironments of cells or tissues. In FCS, fluorescence intensity fluctuations due to Brown movement are measured in a microscopic detection volume defined by a tightly focused laser beam. By fluorescence correlation analysis, a multitude of parameters such as local concentrations & diffuse coefficients are assessed. Combined with high sensitive photoelectric detector, processing circuit and high performance computer, the applications has been reported includes combination of antigen and antibody, micro PH measuring, pharmaceutical drug screening and so on. Particle in light field will be applied a radiation force, especially in two-photon excitation FCS due to its high intensity focused laser beam. This radiation force can be described by two components, i.e. scattering force and gradient force. In this paper, we simulate the two forces of the particle applied in Rayleigh scattering regime, and analyze the radiation force influenced by different radius of the particle.