Abstract: The film-forming mechanism of stearic acid modified SnAgCu alloy powders by vacuum evaporation was researched. Scanning electronic microscope (SEM) and transmission electronic microscope (TEM) were used to observe the morphology and structure of the coated powders. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were adopted to test the transmission absorption spectrum and photoelectron energy of the coated powders. The results indicate that uniform and compact coatings whose thickness was 5 to 10 nm could be obtained. The growth mechanism of stearic acid coatings on SnAgCu alloy powders was a process of physical adsorption and followed the Volmer-Weber model. The essence of the process was the transformation between vapor and liquid and the growth of crystals.