Using plane element and three-dimensional element of finite element method to calculate stress concentration factor (SCF) of symmetrical U-shaped notch sheet under tensile load respectively, a conclusion is obtained that the computing results of the plane element are always lower than those of the three-dimensional element. On the basis of this, a comparison is done between the results of three-dimensional element and the computing results from Neuber's formula and from Barrata-Neal's formula. It is found that, compared with the FEM results of the 6 kinds of notched sheet, the deviation of the results obtained using Neuber's formula ranges from 8.5% to 13.3%, and the maximum deviation of SCF got by Barrata-Neal's formula is 5.5%. In view of these facts, the results from Neuber's formula and from Barrata-Neal's formula are modified using FEM results，what is more, the modified formulae for estimating SCF of U-shaped notch sheet under tensile load are obtained.