Hydroxyapatite (HA) is a useful substance because of its biocompatibility and adsorption capability. Tubular HA particles are highly attractive as novel tissue scaffolds and as drug carriers because of their hollow structures. We previously found that tubular HA particles were formed when a mixture of α-tricalcium phosphate (α-TCP) and anatase was hydrothermally treated. However, the formation mechanism is still unclear. In this study, we investigated the effect of anatase on the formation of tubular HA particles. The formation of tubular HA particles was enhanced when a pellet of a mixture of α-TCP and anatase was UV-irradiated before hydrothermal treatment. The tubular HA formation was observed only when anatase particles with adequate size were added in adequate quantities. We found that the surface properties, size, and quantity of the anatase particles were important for the formation of tubular HA particles. The tubular HA particles were not formed in the early stages of the reaction, they were formed only after some crystal growth had occurred. The anatase particles controlled the nucleation and crystal growth of HA.