The concept of “effective width” of plates subjected mainly to uniaxial compression is widely used in design of plate structures. In this paper, an analytical expression for effective width of rectangular plates subjected to uniaxial compression is derived taking account of initial deflection and welding residual stresses. In order to verify the validity of this expression, it is used to evaluate the ultimate strength of compressed plates. This is then compared with experimental results presented by Ueda et al. and Dwight et al and found to be in good agreement. An expression is also derived to evaluate the effective width of rectangular plates subjected to biaxial compression and shear taking initial deflection and welding residual stresses into consideration. Comparisons with available results of finite element analysis and experimental results exhibits the accuracy and reliability of this expression.