Segregation phenomena occurred from variations of fine particle fractions(0.359mm, 0.194mm) in a 0.109m ID fluidized bed of coarse particle(0.715mm) were measured by a differential pressure transdu-cer and were analyzed by the standard deviation and mean pressure of pressure fluctuation. The variations of minimum fluidizatiion velocity according to fine particle fractions were investigated and the fluidized bed behaviors such as bubble motion and particle mixing were interpreted by the standard deviation. The method for determination of a minimum fluidization velocity from the linear relationship between standard deviation and air velocity which has been applied only for the uniform sized particle system could also be applied in the binary syste. segregation phenomena occurred by the difference of particle size could also be predic-ted by the pressure fluctuations.