In general, when a shaft having general cross-section other than circular cross-section is subjected to torsion, the cross-section will no longer be planar surface as the torsional deformation increases, and it is necessary to consider warping in the cross-section. Hence, the torsional theory for a shaft having general cross section was proposed by Saint-Venant and has been widely used for many years. However, Saint-Venant's torsion theory assumes that deformation is relatively small, and it is not necessarily valid for large torsions. Therefore, in a series of our studies, the strain distributions under large torsion of rubber shafts having a square cross-section and rectangular cross-sections with different aspect ratios had been investigated by using the image analysis based on the Natural Strain theory proposed in this research. Considering the number of sides in cross-section, it can be imagined that as the number of sides decreases, the difference in strain distribution compared with a circular cross-section will become greater. Hence, in the present study, the strain distribution of a triangular cross-section, which has the fewest number of sides, is investigated experimentally and compared with the strain distribution of a square cross-section. Consequently, it is revealed that the difference of the extensional strain between the outer edge and the mid‑span of the side in the equilateral triangular cross-section is greater than that of the square cross-section. On the other hand, the difference of the shear strain of the equilateral triangular cross-section is smaller than that of the square cross-section.