In this study, ultra fine B4C-SiC composite powder was prepared by mechanically activated self-propagating synthesis (MASHS) method from the raw material of B2O3, Si, C and Mg. Initially the synthesized samples contained MgO, B4C, SiC and a small amount of magnesium borates, residual carbon and silicon. XRD analysis after acid leaching of combustion products showed a great effect of acid leaching on removing the impurities. The remaining substances significantly reduced by increasing the energy of milling. The comparison of diffraction patterns after synthesis and after acid leaching showed that acid leaching had moved the peaks to the lower angles. This is due to the development of solid solution and a decrease in contraction stress during the synthesis. Optimum sample prepared with milling parameter of ball to powder ratio of 20:1, rotation speed 300 rpm and milling time of 12 h and synthesized at 1000 degrees C furnace temperature. The average crystallite size of B4C and SiC were calculated less than 15 nm. These values are consistent with the TEM results while most of grain size was less than 30 nm. The particle size analysis of optimum sample indicated that 99.7% of synthesized nanocomposite agglomerated particles were less than 1000 nm. (C) 2015 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.