The poly(methyl methacrylate)/hydroxyapatite (PMMA/5HA) composites were prepared by using heat-processing polymers powder and liquid method with a ratio of 10 : 4 according to the dental laboratory practice. The hydroxyapatite (HA) was surface treated using 3-(trimethoxysilyl) propyl methacrylate (gamma-MPS) with different concentrations based on HA (i.e., 2, 4, 6, and 8%). The chemical treatment of HA was examined using Fourier Transform Infrared Ray Spectroscopy (FTIR). The mechanical properties of PMMA/5HA composites were characterized using tensile, 3-point bending flexural and fracture toughness tests. The morphological properties of the PMMA/5HA composites were characterized using field emission scanning electron microscopy (FESEM). The intensities of absorption bands at 840, 873, and 1031 cm(-1) which are corresponding to Si-O stretching were observed and became higher as the increasing concentration of gamma-MPS for the HA treatment. This indicates that the HA was successfully treated by gamma-MPS. It is interesting to note that the flexural modulus and strength of PMMA/5HA was increased significantly by 6% gamma-MPS. The improvement of the mechanical strength could be related to the enhancement of interfacial interaction between PMMA and HA by the surface treatment of gamma-MPS. In addition, the fracture toughness properties of PMMA/5HA composites were also influenced by the different concentration of gamma-MPS. It is believed that the gamma-MPS could act as a good silane coupling agent for the PMMA/5HA composites. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 218-228, 2010