The catalyst dosage in cloud seeding depends on the fields of temperature, water vapor, flow, background number concentration of hydrometeors, and spectral distributions of various particles in the seeded cloud. The formula for estimating the catalyst dosage of aircraft cloud seeding in stratiform clouds is put forward, on the basis of the supercooled cloud water content, saturation vapor density difference between ice and water surface, average accretion mass of ice crystal groups, effective diffusional scope of a catalyst, and nucleation rate. The equivalent masses of 50, 100, 150 μm for rain drop diameters are selected to represent the average accretion mass of ice crystal groups under different nucleation accretion mechanisms. For the homogeneous nucleation, the nucleation rate is taken as a constant of 1012 g-1, and for heterogeneous nucleation, taking the nucleation rates at temperature of －6 ℃，－8 ℃，－10 ℃ as examples, the dosages of catalyst are calculated according to the proposed formula. The estimated result agrees well with the widely used dosage according to foreign experiences. Also it is found that the dosage is obviously lower for most aircraft precipitation enhancement operations in China, compared with the estimated and recommended dosages.