PURPOSE: To produce magnesium oxide high in fluidity, high in hydration resistance, excellent in work efficiency and having such aggregated particle diameter and bulk density as to enable high filling into resin required to well improve heat conductivity by firing at a temp. lower than that of the conventional method.

CONSTITUTION: High dispersibility magnesium hydroxide having ≤2μm average primary particle diameter, ≤2μm average aggregated particle diameter and 1-20m2/g BET specific surface area is fired at 1,100-1,600°C and the resulting fired body is crushed and classified to produce the objective magnesium oxide having about ≤20μm average aggregated particle diameter.