Tantalum diboride (TaB2) was deposited on a quartz substrate from a gas mixture of TaCl5, BCl3, H2, and Ar at a temperature between 900 and 1300° C. When the atomic ratio (B/Ta) in source gas was held above 1.0 at 1000° C, TaB2 with a composition of between TaB1.90 and TaB1.95 was obtained in a single phase. The deposits grew to grain crystals with an increase in temperature and with an increase in the atomic ratio (B/Ta) in the source gas. The mass transfer of TaCl5 was supposed to be the rate-determining step. The Vickers microhardness values for the coating deposited at 1100° C from a source gas with atomic ratio (B/Ta) above 1.0 were 3500 to 4100 kg mm−2. Dispersing Ni or Pd on the substrate as an impurity, woolly crystals of up to 100μm in length were grown in 30 min at 1050° C, and the growth mechanism was thought to be that of tip-VLS.