The influence of the alkyl chain length of the ionic liquid surfactants 1-hexadecyl-3-alkyl imidazolium bromide [C16imCn]Br (n = 2-16) on their aggregation behavior with sodium dodecyl sulfate (SDS) in water was studied. The rheological properties, thermostability, and microstructure of the samples were characterized via a combination of rheology, cryo-transmission electron microscopy, polarization optical microscopy, and small-angle X-ray scattering. Upon the addition of SDS, the [C16imCn]Br (n = 2, 4, 6) rodlike micelles transit into the gels with high water content. The effects of molar ratio and alkyl chain length on the viscoelasticity and thermal stability of the SDS/[C16imCn]Br (n = 2, 4, 6) gels were studied. However, the [C16imCn]Br (n = 8, 10, 12, 14, 16) rodlike micelles precipitate with the addition of SDS. The [C16imCn]Br (n = 10, 12, 14, 16) gels transit to the rodlike micelles with the proper addition of SDS. The mechanism of the influence of the alkyl chain length of the [C16imCn]Br on their aggregation behavior with SDS was proposed.