Hydroxyapatite (HAp) crystals were grown from simulated body fluid (SBF) onto the arachidic acid monolayers with carboxyl groups, which were prepared by the Langmuir-Blodgett (LB) method on substrates. From X-ray diffraction patterns, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and infrared spectra, it was concluded that the crystalline phase nucleated on the LB monolayers was HAp, which was induced by the monolayer because no crystalline materials were grown on substrates without a monolayer, According to SEM and TEM observations, HAp precipitates were hemispherical aggregates consisting of the plate-like crystallites whose specific {100} surfaces were well developed. The HAp crystallites were preferentially oriented with their c-axes in a specific direction parallel to the substrate near the inorganic/organic interface, which suggests the control of the crystallographic orientation in HAp by the aligned carboxyl groups in the LB monolayer.