Vacuum aluminothermic reduction of magnesia is multi-phase chemical reaction and the briquette forming condition has a great influence on the extent of the reactions. This paper focused on the effect of briquette forming condition on the extraction of magnesium from calcined magnesite. Briquettes were prepared at different conditions, including two forming pressures, four briquette thicknesses and two briquette diameters. The reduction ratio was calculated from the weight loss of the briquette before and after thermal reduction. The obtained condensed magnesium and briquette residues were mainly characterized via X-ray diffraction analysis and scanning electron microscopy. It was revealed that the briquette preparation condition had a great influence on the reduction ratio of magnesia and the phase constitution of the briquette residues, but had a minor influence on the morphology of condensed magnesium.