The effect of a different hydrocarbon chain length of carboxylate acid, i.e. dodecanoic acid, ${\mathrm{CH}}_3({\mathrm{CH}}_{}{)}_{10}\mathrm{COOH}$ or lauric acid and hexadecanoic acid, ${\mathrm{CH}}_3({\mathrm{CH}}_2{)}_{14}\mathrm{COOH}$ or palmitic acid as a co‐surfactant in the 0.3 M sodium dedecyl sulfate, SDS micellar solution has been studied using small angle neutron scattering (SANS). The present of lauric acid has induced the SDS structural micelles. The ellipsoid micelles structures changed significantly in length (major axis) from 22.6 Å to 37.1 Å at a fixed minor axis of 16.7 Å in the present of 0.005 M to 0.1 M lauric acid. Nevertheless, this effect did not occur in the present of palmitic acid with the same concentration range. The present of palmitic acid molecules performed insignificant effect on the SDS micelles growth where the major axis of the micelle was elongated from 22.9 Å to 25.3 Å only. It showed that the appropriate hydrocarbon chain length between surfactant and co‐surfactant molecules emerged as one of the determining factors in forming a mixed micelles structure.