Silver plate was found to prevent spoilage of foods thousands year ago and Ag nanoparticle is known to have great antibacterial activity. In this study, Ag nanoparticles with low than 10 nm average diameter were prepared in the present of glucose. A hypothetical "molecular cage" was built by glucose molecules in solution to limit the chemical reaction in a tiny space and to control the size and morphology of Ag nanoparticles. A possible synthesis mechanism of Ag nanoparticles in confined space was proposed. Escherichia coli and Staphylococcus aureus were used to investigate the antibacterial activity of the synthesized Ag nanoparticles. Compared with the Ag nanoparticles obtained by conventional chemical precipitation method, the Ag nanoparticles synthesized in our study have better antibacterial activities for both Escherichia coli and Staphylococcus aureus in low concentrations.