Abstract: An incompatible element with quadratic completeness in the physical space is developed based on strain gradient theory. After verifying the element, scale effects of ultra-thin beams' bending under the uniform load are studied with this element. Numerical results agree with the experiment observations that the thinner the bean, the more significant the scale effects. Results also show that the scale effect in plane stress state is slightly stronger than that in plane strain state. In the end, it is indicated that the bending stiffness increases when the beam's thickness increases.