Nano/micro-sized calcium phosphate (CaP) coating was prepared on carbon/carbon (C/C) composites by ultrasonic assisted electrochemical deposition. The coating obtained at different deposition voltages contained a mixture of hydroxyapatite (HA) and brushite (DCPD). The homogenous coating prepared at 2.4 V consisted of nano-sized and needle-like HA embedded in micro-sized and plate-like DCPD. An interlocking structure was formed along the depth direction of the coating. The internal stress may be released effectively through the interlocking structure of the coating. And the plate-like crystals of the coating were inset in the grooves on the surface of C/C composites. This led to a better adhesive strength of the coating. Meanwhile, the formed interlocking structure could help enhance cohesive strength of the coating. It was found that the growth of CaP crystals in the coating under the voltage of 2.4 V consisted of the plate-like crystals deposited initially. Then the plate-needle-like crystals of submicron size formed among the plate-like crystals and developed needle-like ones. The CaP-coated C/C composites might improve the biological properties of coating for its unique morphology, structures and strong adhesion to the C/C substrate. Copyright © 2011 John Wiley & Sons, Ltd.