Nacre of molluscan shells has evolved through millions of years to a level of perfect microstructure and excellent mechanical properties. Compressive strengths of nacre parallel and vertical to the surfaces of platelets are tested as 420MPa and 500MPa, respectively. The fracture surfaces are characterized by SEM and the fracture processes are analyzed. The toughening mechanism of compressive loading parallel to the aragonite platelet mainly includes cracks deflection, platelets disengagement, platelets crunch, platelet interlock, organic matrix bridging and mineral bridge. Comparably, the toughening mechanism of compressive loading vertical to the aragonite platelet mainly includes platelets crunch, organic matrix deformation and mineral bridges collapse. Additionally, the propagation of crack deflections in compressive loading parallel to the aragonite platelet is different from that in bending fracture of cross section.