Theoretical Modelling of rectangular concrete beams reinforced internally with Glass Fibre Reinforced Polymer (GFRP) reinforcements under pure torsion is carried out in this study. Different parameters like grade of concrete, beam longitudinal reinforcement ratio and transverse stirrups spacing are considered. The basic strength properties of concrete, steel and GFRP reinforcements are determined experimentally. Theoretical torque verses twist relationship is established for various values of torque and twist using elastic, plastic theories of torsion. Finally the ultimate torque is determined using space truss analogy and softening truss model for different parameters and based on this study, a good agreement is made between the theoretical behaviour GFRP reinforced and conventionally reinforced beams.