A method for calibrating the optical-fiber probe has been developed with adequate confidence. The distribution of solids volume concentration was then measured in a gas-solids flow at high solids loading ratio (i.e. solids/gas mass flowrate ratio) up to 70 in a pneumatic horizontal system with 18 mm and 30 mm i.d. pipelines. The local volume concentration of solids near the pipe bottom is well known to be smaller than its concentration in the loosely packed bed and to increase slowly with increasing solids loading ratio and decreasing pipe size, whereas the solids in the upper part of the pipe tend to concentrate to form a higher local concentration, probably because of electrostatic effects. However, the validity of the model of Hong et al., [6] is reasonably well supported by the experimental average volume concentration of solids.