In this work, author tried to determine the rate of lateral solid mixing in gas-solid fluidized bed, using sodium chloride particles as a tracer. The rate of solid mixing of different particle diameters(100∼140, 60∼140, 60∼100 mesh) has been observed to depend upon gas velocity, and the empirical correlation has been found as follow: W=a exp(bu), a,b=const. Based on the rate of mixing determined in small scale apparutus, author proposed one of convenient scale-up procedure considered to be very important in the design of industrial scale plant.