The preparation of monodispersed inorganic nanoparticles is of great interest for many applications. In this article, transparent oil dispersion of monodispersed amorphous CaCO3 nanoparticles with high concentration and long-term stability were controllably prepared by a reverse microemulsion method. The effects of the addition amount of extra minute water and (NH4)(2)CO3 as novel assistant promoter, reaction temperature, Ca(OH)(2)/CaO mole ratio were explored. The optimum synthesis conditions were achieved. The as-prepared transparent oil nanodispersion had a good monodispersity, a uniform particle size of 8-10 nm, a high stability of over 12 months, a high solid content of 38 wt% (Ca content of about 15.5 wt%) and a high total base number of 416 mgKOH/g. The preparation process was further investigated by polarized optical microscope and Fourier transform infrared. This nanodispersion will find a promising applicability as an excellent nanodetergent in the fields of automobile and marine lubricants. (C) 2017 American Institute of Chemical Engineers