This paper highlights the effect of tension stiffening index on the torsional resistance at the crack and peak loads in under-reinforced fibrous normal strength concrete beams under pure torsional loading. The different grades of steel reinforcements were used with different surface texture of the reinforcements. Four fibrous normal strength concrete beams were cast and tested under pure torsional loading. The beams were designed based on ACI 318, where the clear span to depth ratio and aspect ratio of the section were kept constant at 5.72 and 1.22, respectively. The test results verified that the torsional resistance of peak load was improved up to 12.91% due to the reduction of tension stiffening index, whereas the cracking torsional resistance and the value of twisting angle were slightly influenced. Moreover, due to the reduction of tension stiffening index up to 11.66%, the strain in transverse and longitudinal reinforcements were enhanced up to 91.8% and 73.6%, respectively.
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