Abstract: Ultrafine barium carbonate （BaCO<SUB>3</SUB>） prepared with coprecipitation by a micro-reactor was researched.Effects of flow rate,reaction temperature,concentration,and dispersants on size and morphology of ultrafine BaCO<SUB>3</SUB> were investigated.BaCO<SUB>3</SUB> particles were characterized by SEM,LGI,and BET.Results showed uniform columnar ultrafine BaCO<SUB>3</SUB> particles with slenderness ratio of 4 and spherical particles with average size of about 300 nm can be obtained by micro-reactor.