The reduction kinetics of the U3O8 via AUC( Ammonium Uranyl Carbonate) has been investigated. The results showed that the reaction rate was independent of particle size and the conversion had a linear relationship with time. It suggested chat the particle was highly porous and was composed of plate-like "grains". This analysis was confirmed by observing the surface and cross section of the particles with SEM. It provides the physical basis for establishing the reaction models of highly reactive powders and their pellets. The calculated curves from the models by using parameters determined independently were in good agreement with the experimental results.