The formation of a cationic-lipid/DNA complex (lipoplex), where the lipids that have a quaternary amine headgroup and alkyl tails of various lengths are newly synthesized, was investigated by use of isothermal titration calorimetry (ITC) and small-angle X-ray scattering (SAXS). SAXS showed that all of the samples formed a lamellar structure with DNA sandwiched between the lipid bilayers. For the hexyl tail, ITC showed an exothermic response, while for the nonyl and dodecyl tails, it was endothermic. We propose a lipoplex formation model to explain both SAXS and ITC results.