Abstract: Acinetobacter baumannii has emerged as one of the leading bacteria for nosocomial infections, especially in burn wards and ICUs. The bacteria can easily form biofilm and readily attach to abiotic and biotic surfaces, resulting in persistent biofilm–mediated infections. Being surrounded by self–produced extracellular polymeric substance (EPS), the microorganisms in biofilm can acquire protective property against detrimental environment and their tolerance toward antibiotics is increased. Poly–β–1–6–N–acetylglucosamine (PNAG), the common constituent of EPS in Acinetobacter baumannii, acts as the key virulence factor and plays a crucial role in biofilm formation process. This review describes the properties and functions of the PNAG and its influence on biofilm formation and drug resistance of Acinetobacter baumannii.