Hydroxyapatite (HAp) and biphasic tricalcium phosphate (BCP) nanopowders were synthesized by the microwave assisted process and their microstructures and crystal structures were investigated depending on the pH of the starting solution. The synthesized powders were spherical in shape and the average size was in the range of 50-90 nm in diameter. In the range of pH 6-12, HAp and beta-TCP phases coexisted. The yield of HAp increased remarkably as the pH of the solution increased while alpha-TCP and beta-TCP yields decreased. At pH 12, monolithic HAp powder was synthesized with nanoparticles. From the SBF experiment, the maximum resorption of Ca2+, was detected in the powders synthesized at pH 6. However, at pH 12, the monolithic HAp powder showed high stability in the SBF (c) 2007 Elsevier B.V. All rights reserved.