The phase behaviors of the complex formed by didodecyldimethylammonium bromide(DDAB)and cetyltrimethylammonium bromide(CTAB)interacting with three different types of DNAs,salmon testes DNA(~2000 bp),21-bp double-stranded oligonucleotides(oligo-ds DNA),and 21-nt single-stranded oligonucleotides(oligo-ss DNA)were studied by synchrotron small-angle X-ray scattering.It was found that the DNA length and flexibility,together with the positive/negative charge ratio,determined the final structure.At higher charge ratios,the DNA length exhibited negligible effect.Both oligo-ds DNA and salmon DNA formed inverted hexagonal packing of cylinders with CTAB,as well as bilayered lamella with DDAB.However,at lower charge ratios,oligo-ds DNA formed a distorted hexagonal phase with CTAB and a new structure with DDAB,which was different from the behaviors of salmon DNA.The flexible oligo-ss DNA formed rich structures that were subject to environmental disturbance.Kinetic study also indicated that the structures of the complex formed by oligo-ss DNA took much longer to mature than the structures formed by oligo-ds DNA.We attributed this result to the conformational adjustment of oligo-ss DNA in the complex. The phase behaviors of the complex formed by didodecyldimethylammonium bromide (DDAB) and cetyltrimethylammonium bromide (CTAB) interacting with three different types of DNAs, salmon testes DNA (~ 2000 bp), 21-bp double- stranded oligonucleotides and 21-nt single-stranded oligonucleotides were studied by synchrotron small-angle X-ray scattering. It was found that the DNA length and flexibility, together with the positive / negative charge ratio, determined the final structure. At higher charge ratios, the DNA length illustrated negligible effect. Both oligo-ds DNA and salmon DNA formed inverted hexagonal packing of cylinders with CTAB, as well as bilayered lamella with DDAB. Less, at lower charge ratios, oligo-ds DNA formed a distorted hexagonal phase with CTAB and a new structure with DDAB, which was different from the behaviors of salmon DNA.The flexible oligo-ss DNA formed rich structures that were subject to environmental disturbance. Kinetic study also indicated that the struggles ctures of the complex formed by oligo-ss DNA took much longer to mature than the structures formed by oligo-ds DNA. We attributed this result to the conformational adjustment of oligo-ss DNA in the complex.