Brownian Motion is one of the important characteristics of submicron particles.On the basis of observatiorl for Brownian motion of particles,the authors present a medel to describethis physieal process by usin g fractal theory a nd this medel is proved to be valid in theory.Throughfurther an alyzing,we establish the relationship between fractal characteristics and particle sizeparameters.Combining image signal processing technique,particle size information can be obtainedby computing fractal dimension values,and then a new method for particle sking will be developedinfuture.