The growth mechanism of an "in-gel synthesis method", that is, the effects of composition and structure of the lamellar gel phase below the Krafft temperature of surfactant solutions on the growth of long gold nanorods, was investigated. We changed the alkyl chain length of surfactant molecules to investigate the effect of surfactant self-assembly on the elongation of gold nanorods systematically; eight mixed solutions of alkyltrimethylammonium bromide (C nTAB; n = 2-16; n = even) with C18TAB were used for investigation. The Krafft temperature, self-assembly of surfactant molecules, and the crystallization process of each mixture were observed by differential scanning calorimetry, wide-angle X-ray scattering, visual inspection, and small-angle X-ray scattering. Gold nanorods were synthesized in these eight surfactant mixtures. These observations demonstrated that when the surfactant Lβ phase sustains for a long time, the space of the water layer is also kept large enough for the seeds to take up Au ions bound to surfactant micelles. In this case, the seeds can form long nanorods between bilayers. We conclude that not only the stability of the lamellar gel phase but also co-existence of Au-ion carriers, that is, surfactant micelles, is essential for the elongation of long gold nanorods.