The torsional rigidity of spun yarn is derived in terms of physical and mechanical characteristics of its constituent fibres and yarn structural parameters to investigate its torsional behavior. The torsional rigidity of spun yarn increases with fibre length, shear modulus and the ratio of bending rigidity to the torsional rigidity of its constituent fibres. Maximum yarn torsional rigidity was obtained with increasing yarn surface helix angle. However, the theoretical torsional rigidity of the spun yarn increases with the coefficient of interfibre friction.