Ultrafine TiB2 powders were synthesized by rapid carbothermal reduction in a vertical tubular reactor through which the particles fell freely. The starting materials were TiO2, H3BO3 and cornstarch, which were mixed and calcined at 400 degrees C for 1 h. The calcined precursor was milled, sieved and then fed into the top of the reactor. The reduction was carried out between 1786-1791 degrees C in a downward flow of argon. Product particles were recovered at the bottom of the reactor. When the precursor molar composition ratio was TiO2 : B2O3:C = 1:2:5.5, the carbon content in the product was 5 wt % and the crystallite size was 80 nm. The carbon content in the product was reduced to 2.9 wt % by a heat treatment in an H-2 atmosphere for 9 h.