The structural changes of wormlike micelles composed of Tween 80 and Brij 30 in the presence of a common ionic liquid 1-butyl-3-methylimidazolium tetralluoroborate (bmimBF(4)) were studied. Upon addition of bmimBF4, the viscosities of the wormlike micelle systems decrease, or in other words, more Brij 30 is needed to attain a similar viscosity value. The changes of the storage/loss modulus and the relaxation time also show that the viscoelastic properties of the wormlike micelles decrease with increasing bmimBF4 concentration. We believe that these results are attributed to a wormlike-rod/spherical micellar transition in the wormlike micelle solution, which is further confirmed by freeze-fracture transmission electron microscopy (FF-TEM) images. Additionally, the location of bmimBF4 in mixed micelles was also studied to explain the structure transition in the above systems.