Abstract: This investigation highlights the influence of double idealized shear flow zones on the torsional resistance for under-reinforced fibrous normal strength concrete beams subjected to pure torsion. The beam sections were double reinforced by transverse and longitudinal reinforcements, once was located in the idealized shear flow zone and the other one was located in the idealized tension zone. To study the influence of double idealized shear flow zones on the torsional resistance of under-reinforced fibrous normal strength concrete beams, four fibrous concrete beams were cast and tested under pure torsion. The under-reinforced beams designed based on ACI 318-14. The main transverse and longitudinal reinforcements in the shear low zone are kept constant in the beams while the transverse reinforcement in the idealized tension zone was covered different percentage of this zone. The test results were confirmed that the torsional resistance at peak loads was improved up to 23.7% while the area enclosed by stirrup in core zone has extended to 46.2%. In addition, the twisting angle at peak loads was enhanced up to 60.9%and the shear strain in concrete and the strain in longitudinal reinforcement were enhanced whereas the strain in transverse reinforcement was decreased. The spacing between spiral cracks and the inclination angle of crack at failure were increased with inclusion of reinforcement in core zone. The space truss model was modified to cover the influence of double shear flow zones. The proposed modification model was showed that it has a good agreement with test results.