The effect of MgO diluents in combustion synthesis of $TiB_2$ nano particles was investigated. The reaction $TiO_2;+;B_2O_3;+;xMg;{ ightarrow};TiB_2;+;5MgO$ was used to synthesize $TiB_2$ nano particles. The combustion velocity was measured to examine the relation between the reaction temperature and the morphologies of particles. The diluent MgO did not react with the reactants and played a role to decrease combustion temperature. As the MgO diluents contents increased, the particles with the smaller size and the narrower size distribution were synthesized. At the condition of 6 mole of MgO, the combustion wave velocity was about 5 cm/see and the synthesized particles showed the size of 60 nm with narrow size distribution.?????????????T散????