In this study, a simple one-step wet chemical method was presented to prepare colloidal cerium oxide, CeO2, nanocrystals. By a direct decomposition of cerium(IV) ammonium nitrate in a solution of 1-octadecene and oleylamine at 180 degrees C, well-dispersed CeO2 nanoparticle colloid, with an average size of about 4.5 nm, was obtained.