The size of a particle smaller than the diffraction limit is measured using a conventional optical microscope by
adopting a standing wave evanescent field illumination. The scattering intensity from a nanoparticle is periodically
modulated by shifting the intensity fringes of standing evanescent field. By measuring the intensity variation of scattered
light during one cycle of modulation, particle sizes can be easily estimated. Furthermore, this technique has weak
dependence on the material of particles. From the experimental result, the particle size ranging from 20 to 250 nm is
successfully determined. This technique offers a low-cost size measurement for nanoparticles.