An outbreak of norovirus GII.4/Sydney_2012 affected a China elder care facility in December 2012. A total of 39 elderly people and staff met the outbreak case definition. The attack rates in the elderly and the staff were 15.9% (31/195) and 23.2% (19/82), respectively, including 13 asymptomatic cases in the staff. The result of gene sequencing revealed that the outbreak was caused by norovirus GII.4 Sydney. The mode of transmission of this outbreak was proven to be person-to-person. The first case (a self-cared elder) was affected outside the elder care facility and was not isolated after returning. Norovirus was transmitted via close contact among the self-cared elderly. Then, through service-related close contact, the attendants promoted the cross-transmission between the self-cared elderly and the nursed elderly. The virus was also spread among the staff via daily contact. In the elder care facility, the asymptomatic cases in the attendants played an important role in the transmission of norovirus, which deserves high attention.