Diborides, including zirconium diboride (ZrB2) and titanium diboride (TiB2), have a number of desirable ceramic qualities that make suitable for preparing ceramic-matrix composites. However, synthesizing a composite based on these materials frequently requires high temperatures and complex synthetic methods. In this study, a nanocrystalline ZrB2-TiB2 powder was synthesized via mechanical alloying (MA) of mixture of elemental Zr, Ti, and B powders mixed at a Zr/Ti/B mole ratio of 1:1:4, a 10:1 ball-to-powder weight ratio, and 500 rpm rotational speed in a planetary ball-mill under argon atmosphere using a ZrO2 vial and balls. MA has the benefits of low-cost, simplicity and high throughputs. The effect of milling time on the phase change was investigated by X-ray diffraction (XRD), and the microstructure development of the powder mixture was monitored by field emission scanning electron microscopy (FESEM) and transmission electron mi-croscopy (TEM). It was found that after 120 h of milling, a nanoscale composite powder with a ~20 nm mean particle size was obtained. Moreover, the TEM examination clearly showed the composite formed between the nanoscale TiB2 and ZrB2 particles. Finally, the milling mechanism was discussed, illustrating the time needed to initiate the reaction and when the fine, homogenous final composite was produced.