In the paper, mechanochemical decomposition (a combine of mechanical milling and chemical decomposition) on bastnaesite concentrate with NaOH solution was studied. Particle size, BET specific surface area and pore volume were measured to study the structural developments of decomposition residue containing activated bastnaesite and decomposition product RE(OH)(3). X-ray diffraction analysis and SEM-EDS were used to investigate the microstructural changes. The results showed that at the early stage, breakage from balls to particles played an important role. The particle size in initial bastnaesite decreased rapidly from 125 mu m to 7.25 mu m and specific surface area increased remarkably from 0.04 m(2)/g to 7.33 m(2)/g in the first 5 min. The bastnaesite particles with fresh surface could directly react with NaOH, and increase in amorphization degree obviously improved reactivity of bastnaesite. At the later stage, with progressive decomposition, the thickness of product layer increased and the structure became dense with decrease in the pore volume from 0.031 cm(3)/g to 0.024 cm(3)/g. In addition, this is accompanied with increase in the crystallinity degree and crystallite size as well as decrease in the microstrain. Thus, the decomposition of bastnaesite can be enhanced. Washing efficiency of fluorine reached 97.6% at 180 degrees C using 56% of NaOH concentrate with 0.8:1 of liquid-solid for 180 min. Finally, the change of fluorine washing efficiency with time further verified the result that the reaction controlling step had a transformation from chemical reaction to diffusion control through product layer.