The influence of reaction temperature, time and the amount of reactant NaBH4 on the formation of ZrB2 were investigated. Reaction (1) is thermodynamically spontaneous (calculated ΔG=−440.85 kJ at 500 °C) and exothermic (calculated ΔH=−75.79 kJ at 25 °C). When the temperature was higher than 500 °C, the crystallinity and the crystallite size both increased with increasing temperature. ZrB2 was not formed when the temperature was lower than 450 °C. The reaction was usually incomplete and the crystallinity was very poor when reaction time was less than 6 h. However, varying the reaction time between 12 and 24 h did not significantly affect the crystallinity or the crystallite size. When ZrCl4 reacted with NaBH4 with a molar ratio of ZrCl4/NaBH4 = 1/2 or 1/4, the product contained both ZrB2 and ZrO2 and the crystallinity was poor. Only with the molar ratio of ZrCl4/NaBH4 = 1/8, pure ZrB2 was obtained. With the smaller ZrCl4/NaBH4 ratios, the ZrCl4 did not react completely and would hydrolyze when the product washed with distilled water. With the molar ratio of ZrCl4/NaBH4 = 1/8, the maximum pressure is about 18–22 MPa in the temperature range of 500–700 °C, which is estimated according to the amount of hydrogen treated as an ideal gas. It is believed that the increasing hydrogen pressure in the autoclave may be beneficial to the formation and the crystallinity of ZrB2.