The application of CaO refractories was limited due to their poor hydration resistance. Introducing appropriate additive is an effective way to improve their hydration resistance. Additive WO3 was introduced in the preparation of CaO aggregates using a calcination method in this study. The introduction of WO3 decreased the apparent porosity of CaO aggregates and promoted the grain growth of CaO by the formation of liquid at high temperatures. The average grain size of the specimens with 0.4 mass % WO3 and treated at 1 550 °C was increased to 51.6 μm comparing to 33.5 μm of those without WO3 additive. As a result, the hydration resistance of CaO aggregates was significantly improved. Besides, WO3 sublimated easily, therefore, less WO3 remained after the heat treatment. The further increase of WO3 addition had little effect on the hydration resistance of CaO specimens.