Silver-O-carboxymethyl glucomannan nanocomposites (Ag-CMG nanocomposites) were synthesized via reduction of [Ag(NH3)2]OH by NaBH4 and HCHO reductant agents in the presence of water-soluble carboxymethyl glucomannan as a stabilizer. The characteristics of Ag-CMG nanocomposites were investigated by transmission electron microscopy (TEM) and UV-vis spectra. The results shown that both NaBH4 and HCHO reductant agents could be used effectively for preparing the Ag-CMG nanocomposites, and carboxymethyl glucomannan substrate could be used as a good stabilizer for nano-system. The average particle size of silver nanoparticles was ranged between 2÷25nm and 10÷20nm when reductive agents were NaBH4 and HCHO, respectively. The characteristic surface plasmon resonance band of silver nanoparticles in the two cases centered at about 384÷425nm. In vitro antibacterial activities of Ag-CMG nanocomposites were evaluated against Escherichia coli (Ec); Proteus mirabilis (Pro); Shigella flexneri (Shi); Salmonella typhi (Sal) and Pseudomonas aeruginosa (Pseu), which are Gram-negative bacteria, and Staphylococcus aureusATCC1128 (Sta); Bacillus cereusATCC9946 (Bc); which are Gram-positive bacteria. The results shown that the Ag-CMG nanocomposites could inhibit the growth and multiplication of the tested bacteria, especially toAg-CMG with smaller particles.