Micelles formed above critical micelle concentration(CMC) in surfactant aqueous solutions are homogeneous and transparent. Their existence are not observed by electron microscopy, which is difficult to analyze size and structures. The small-angle neutron scattering(SANS) method are widely used in colloidal systems since it explains their interaction as well as size and structure of association structures formed in surfactant aqueous solution. This study interpreted size and structure of micelles formed in cationic surfactant alkyltrimethylammonium chloride (C_nTAC) by SANS spectra. The relationship of alkyl chain length(n=12, 16, and 18) and size of micelle is changed linearly. The structural transition isn't observed with increasing concentration for each surfactant. However, intermicellar distance from spectra peak point decreases with concentration and almost constant above 250 mM. This means that structural change of micelles is predicted at higher concentration.