Abstract The thermal decomposition process of zinc acetate dihydrate was studied by differential thermal analysis (DTA) and thermal gravimetric analysis (TGA). It is shown that the water of crystallization is removed at below 200 °C. Anhydrous zinc acetate melts at 210–250 °C, and fully decomposes into ZnO at about 400 °C. The Mechanism of decomposition of anhydrous zinc acetate is inferred as a random nucleation mechanism (Avrami equation II), and a mechanism model is established as ( dα/dt) = 1.8 × 10 7 exp(−103 × 10 3/RT)3 (1 − α)-[− ln(1 − α)] 2/3. Pure ZnO ultrafine powders with uniform size of 20–30 nm were synthesized by spray pyrolysis from solution of zinc acetate dihydrate. The product particle morphology and the effect of reaction temperature on the particle properties can be explained by the random nucleation mechanism and model.