Polyanionic DNA interacts with cationic amphiphiles to form electrostatic complexes exhibiting rich self-assembled structures. This type of complex has been considered as a nonviral carrier in gene therapy and as a template for nanostructure construction. Here we report a thermally-induced phase transition of the complexes of DNA with the mixtures of a cationic surfactant, dodecyltrimethyl bromide (DTAB), and a neutral lipid, dioleoylphosphatidylethanolamine (DOPE), in fully hydrated state. An order-order transition between a multilamellar (Lαc phase and an inverted hexagonal (HIIc) phase was found to occur with the transition temperature adjustable by the DTAB-to-DNA base pair molar ratio (x) and DOPE-to-DTAB molar ratio (m). The stability of the L αc phase was enhanced at lower m and x, as the Lαc-to-HIIc transition temperature increased with the decreases of these two parameters. The suppression of Lαc-to-HIIc transition at lower x was attributed to the lower entropie gain from the counterion release due to the oresence of uncomolexed DNA in the bulk solution.