Abstract：The tensile strength can be determined from the split test of a disk specimen，or Brazil test. However the rock near loading points will yield and fracture，which is different from its testing principle. Two flats compressed in uniform deformation are introduced to the Brazilian disk to improve the stress state. From the results of FEM，the tensile stress in the center of disk decreases with the central angle of the flats，and the ratio of compressive stress to tensile stress increases as well. So it must be studied whether the Griffith criterion in tensile region is true or not true，and how to choose the central angle of flattened disk. The flattened disks of granite，sandstone，lime and diabase with central angle of 20°～90° are tested. The disks do not crack along their symmetric lines，and the tensile strength increases with the central angle of flats，which results from that Griffith criterion gives a higher tensile strength and there is friction effect between flats and the ends of testing machine. The disks crack symmetrically when plastic cushions with thickness of 0.5 mm are used. But the plastic cushions produce new friction in opposite direction，which causes that the strength of disk decreases with increment of central angle. The tensile strengths of circle disk and flattened disk are well matched for granite. The Brazil strength is 13 MPa，and the uniaxial compressive strength is up to 240 MPa. Except the granite，Brazil strengths of the other rocks are much lower than that of the flattened disks，which results from the localized load in Brazil test. In order to reduce the effect of friction and the flat quality of specimen on test results，the better choice of central angle of the flat is 20°～30°。
