The carbothermal reduction synthesis process of titanium diboride (TiB2) particles was studied. In the synthesis of TiB2 using carbothermal reduction from a mixture of TiO2, B2O3 and carbon, solid-solid reactions occur. TiO2 particles rapidly react with carbon to TiC, which then reacts with boron oxide and carbon to TiB2. In the vacuum condition, TiB2 particles were formed within 10 minutes at temperature of 1300oC. It seems that a high exothermic reaction eventually results in the increase of reaction rate. In flowing argon atmosphere, TiB2 particles were formed at temperature of 1550oC after a reaction of 0 minute and it showed a finer particle size than that in the vacuum condition. This is attributed to the faster heat elimination due to the flowing argon. In high atmospheric pressure of argon gas such as 20 atm in reaction or cooling state, the synthesized TiB2 particles shows a mixture of diverse sized particles.