Hydroxyapatite (HA) has great potential as a reinforcing filler for poly (methyl methacrylate) (PMMA) denture base materials. Nevertheless, filler particles need to be homogeneously distributed throughout the PMMA particles to get the maximum benefit from using the filler. Therefore, the physical mixing of the powder components (PMMA and the filler) is strongly preferred to provide the required dispersion of the filler in the matrix. However, conventional techniques that have been tried, such as hand mixing and stirrer mixing techniques, were not effective. Therefore, the current study was designed to experimentally investigate the effect of different mixing times on the fracture toughness of PMMA/HA using a developed ball milling method. In this study, heat cured PMMA reinforced with 15 wt% HA ceramic powder was ground for different times (i.e., 10, 20, 30, and 40 min) via the technique of planetary ball milling (PBM). The ground powder mixtures were used for the fabrication of denture base testing samples. The particle size and distribution of the PMMA/HA composites after milling for several times were determined by the laser light scattering technique. The X-ray diffraction (XRD) patterns of the PMMA/HA composites were obtained. However, no new phase was observed. The effects of mixing time using the PBM technique on the fracture toughness were investigated. The effect of mixing time on the microporosity (voids) on the fractured surface of PMMA/HA was studied with field emission scanning electron microscopy (FESEM). Within the limitation of the current study, 30 min is considered the optimum mixing time for the tested PMMA/HA composite.