Water droplets containing MgCl2 and urea, as the water phase of xylene/water/span-80 water-in-oil (w/o) emulsion system, can act as microreactors where spherical MgO precursor powders with porous microstructure were obtained via homogeneous precipitation. By changing the volume ratio of span-80 to xylene, spherical powders with different sizes were synthesized, and the effect of span-80/xylene volume ratio on powder morphology was examined. The as-prepared powders as well as their precursors were characterized by means of field-emission scanning electron microscopy (FE-SEM), laser particle size analysis (LPS), N-2-adsorption measurements (BET) and thermogravimetry and differential thermal analysis (TG-DTA). The TG-DTA result indicated that the optimal sintering temperature of precursor powders was 450 degrees C. The FE-SEM results showed that the span-80-to-xylene volume ratio had a marked effect on the morphology of powders. When the ratio was 1:40, the FE-SEM and LPS results confirmed the formation of the powder microspheres with a size range of 1-16 mu m, and the surface area of the microspheres was about 75.7 m(2) g(-1) according to the BET result.