The formation of the viscoelastic wormlike micelles in aqueous solution of mixed surfactant system of anionic gemini-type surfactant without a spacer group, disodium 2,3-didodecyl-1,2,3,4-butanetetracarboxylate (GS), and dodecanoyl-N-methyl ethanolamide (NMEA-12) has been studied at 25 °C. With addition of NMEA-12 to the GS–water binary system, a micellar (Wm)–lamellar (Lα) phase transformation takes place at low GS concentration. Within the Wm phase region of GS–NMEA-12–water system, there exists a high viscosity region consisting of a viscoelastic micellar solution of entangled wormlike micelles. On increasing the concentration of GS, micellar growth begins at a slightly lower mixing fraction of NMEA-12. The oscillatory-shear rheological behavior of the viscoelastic solutions can be described by Maxwell model at low -shear frequency region. The increase in viscosity can be simply explained by reducing the effective cross-sectional area per amphiphile upon addition of NMEA-12.