Tests were performed in a batch fast fluidized bed, working at ambient conditions, to check the influence of operational conditions, namely, superficial gas velocity, particle diameter, test duration, and solids inventory, on the size reduction process of a friable material particle. The experimental system consisted of a 0.30 in i.d. vertical riser, 2.22 m high, with primary and secondary air injections, main and secondary cyclones, bag filters, and a solids reinjection valve. Tests were performed with calcined oil shale particles, and the fluidization velocity. ranged from 2.89 to 4.81 m/s. Experimental observation showed the particle, size reduction rate Ito be proportional to the fluidization excess velocity and to mass inventory, as in bubbling fluidized beds. It was observed that a significant amount of material was reduced to fines a - nd that the intermediate size intervals also exhibited some increase in their mass fractions. On this,basis, a model was proposed to simulate the process, considering the existence of two. size reduction mechanisms, namely, abrasive attrition and mechanical fragmentation. The, model predicts the effects of the process conditions and changes in size distribution with time.