Critical velocities were measured by differential pressure transducers in a solid circulation experimental apparatus which was a 1.8 m in length, 0.034 m in diameter of Plexiglas horizontal pipe. Three kinds of particles of FCC catalyst, zinc titanate and polyethylene were used. Pressure drop, standard deviation of pressure drop fluctuations and solid conveying rate were measured to determine the critical velocity. Critical velocity increased with increasing solids conveying rate, however, there was little effect of the pressure on the critical velocity in the range from 1.0 to 2.0 atm. The critical velocity was correlated in terms of particle density, diameter and solid conveying rate. Unstable Pressure balance was occurred at front section and at rear section of a horizontal pipe in a pressurized circulation system when it was disturbed by plugs at a velocity less than U(cr).