A beam theory including the effect of shear deformation is formulated in finite deformation. The governing equations are organized in terms of two non-dimensional parameters, one of which is the slenderness ratio. In order to quantify the contribution of shear, the elastic and inelastic buckling of a simple beam is analysed, and results are examined with respect to those two parameters. The significant reduction of critical stresses is observed for shorter columns, and the range of the slenderness ratio is obtained, in which the difference between this theory and the Bernoulli-Euler beam theory becomes prominent. Moreover, judging from the fact that shorter columns buckle inelastically, this theory becomes more important for the inelastic analyses of such deep beams.