The present study investigates the changes in microstructure and microchemistry (particularly Ca/P ratio) during whisker formation on the surface of TTCP powder in hydrochloric solution. XRD results indicate that the as-fabricated (non-treated) powder has a monolithic TTCP crystal structure. When the powder is treated for 10 min in HCl, a DCPD phase appears on TTCP surface, which increases in intensity with treating time. When treated longer than 4 h, the DCPD intensity decreases. When treated for 24 h, the DCPD phase disappears. TEM results indicate that, when TTCP is treated for 10 min, fine globular-shaped DCPD crystals are observed to precipitate on the surface of TTCP particles. When treated for 1 h, hexagonal Ca-deficient apatite whiskers with an average Ca/P ratio of 1.5 form. With treating time the apatite whiskers continue to grow in size and increase in Ca/P ratio. When treated for 24 h, the length and thickness of the apatite whiskers can reach >500 and >50 nm, respectively, with an average Ca/P ratio of 1.8. The microchemical data indicate that the apatite whiskers grown on the surface of TTCP particles are substantially non-stoichiometric in nature. Compressive strength data indicate that apatite whiskers grown on TTCP surface under a favorable condition can largely improve the properties of the resulting CPC.