Abstract: East China was dominated by a strongly intense, long-duration, and extensive-coverage haze in December 2013. The synoptic characteristics, persistent mechanism, and precursory oceanic conditions for the persistent haze in December 2013 are studied using a monthly NCEP/NCAR reanalysis dataset, particulate matter observations in Huadong, and other regular observational data during 1981-2014. This persistent haze may be closely related to the activity of blocking over East Asia. The blocking over the Bering Sea-Okhotsk Sea, weakened transient eddy over East Asia, persistent trough over Lake Baikal, and attenuated northerly winds over east China would favor a worse diffusion condition for local accumulation and more transportation of particulate matter to East China in December 2013. Moreover, the enhanced blocking over Okhotsk Sea in December 2013 is probably related to the warm sea surface temperature over the tropical Indian Ocean.