Abstract: The total internal reflection velocimetry (TIRV)based on evanescent wave is an ef-ficient method for velocity measurement within a few hundred nanometers to the wall.The eva-nescent wave intensity,I(z),decays exponentially with the distance z away from the wall.When a nanotracer located in this field is illuminated by evanescent wave,its radiation intensity will also obey the exponential decay low,and thus it is possible to determine the nanotracer’s z position by its illuminated intensity.Obviously,one of the key issues of this technology is to determine the base intensity I0 . In this paper,a numerical solution of the nanotracer intensity probability density function is given to predict the intensity distribution according to the exponential decay of the evanescent wave,the tracer concentration distribution and the tracer size variation.By com-paring the measured intensity distribution with the theoretical prediction,the base intensity I0 can be quantitatively determined.Fluorescent polystyrene spheres ofφ100nm andφ250nm are used in the measurements.The validity of this method is verified.The effect of the nanotracer size variation on the determination of I0 is also analyzed.Furthermore,φ100nm nanotracers are used to measure the velocity close to the wall,and the result verifies the validity of our method.