Initial deformation behavior during small punch (SP) testing was investigated in order to estimate yield stress using two sizes of SP specimens, 10 by 10 by 0.5 mm(3) and 10 by 10 by 0.25 mm(3) by finite element (FE) analysis and by experiments with heat-treated SA508 and 12Cr steels. The increase of thickness resulted in a deviation from the initial linearity of the load-displacement curve at higher loads. The deviation was attributed to different causes, namely loss of the constraint try the surrounding material and radial propagation of plastic bending deformation for 0.5 and 0.25-mm specimens, respectively. The causes for the transition of deformation mode were identical irrespective of material strength and work-hardening rate in the range of yield stress about 400 to 900 MPa. Based on the invariance of the deformation mode, a distinct linear relationship between the load at the breakaway from initial linearity and the yield stress was predicted by the FE analysis and compared with the experimental results.