We have studied the formation, structure, and rheological behavior of viscoelastic wormlike micelles in the mixed system of long polyoxyethylene chain phytosterol (PhyEO30) and polyoxyethylene dodecyl ether (C12EOn, n = 3 and 4) surfactants in water. Partial ternary phase diagrams of water/PhyEO30/C12EOn are constructed at 25 degrees C. Addition of C12EOn to the aqueous solution of PhyEO30 in a dilute region increases the viscosity by several orders and forms viscoelastic micellar solution of entangled wormlike micelles showing the Maxwellian behavior at low shear frequencies. With successive addition of C12EOn, ultimately a phase separation occurs with the formation of turbid solution of vesicular dispersion coexisting with the micellar phase. A rapid micellar growth has been observed when C12EO3 replaces the C12EO4. However, no viscosity maximum is seen with the C12EO3 system; the viscosity increases continuously until phase separation. The effect of temperature on water/PhyEO30/C12EO3 system has also been studied. Small-angle X-ray scattering measurements have shown the one-dimensional micellar growth induced by C12EOn and well support the conclusions derived from rheometry.