Hydroxyapatite (HA) and $eta$-tricalcium phosphate ($eta$-TCP) are bio-compatible materials with bones and teeth. HA has been widely applied as bone substitutes because of chemical stability in vivo, while $eta$-TCP has higher resorbability than HA when the material is implanted in a bone defect. In the present study, both HA and $eta$-TCP porous ceramics were soaked in the simulated body fluid in order to investigate the reaction between the materials and the fluid. After the soaking test, carbonate hydroxyapatite was formed on HA surface at 1 week, and then the amount of precipitates increased with increasing period of the soaking test. While $eta$-TCP was not dissolved in the fluid, carbonate hydroxyaopatite was also formed on $eta$-TCP surface after 12 weeks, and the amount of precipitates was less than that on HA. In vitro behavior of HA was similar to that in vivo, but in vitro behavior of $eta$-TCP was not similar to that in vivo.