Abstract: To determine the torsional moment for a statically indeterminate structure, the torsional rigidity of each member is required. This paper presents a procedure for calculating the torsional rigidity of under-reinforced concrete slabs. A typical torque-twist curve was idealized as a polygonal line containing 4 segments, broken at points of initial cracking, absolute cracking and reinforcement yielding. At the uncracked state, St. Venant’s theory was used. The post-cracking torsional rigidity was derived from the rotating-angle softened truss model theory. Comparisons with some published testing results demonstrated that this method can predict “load-deformation” history of under-reinforced concrete slab.