Abstract: By making use of the simultaneous observational meteorological data in 1984 in Shanghai urban and its suburbs, we discussed the formation of urban moisture island and its relationship with urban fog. Three major conclutions are as follows: (1) There are four types of urban moisture island according to formation: a) with less dew deposits in the nighttime, b) with less frost deposits in the nighttime, c) more water evaporation in or after the rain, d) more water evaporation in the fog time. All the above processes are accompanied by the urban heat island. (2) The urban moisture island is a very good indication of radiation fog and advection radiation fog, but there is little relationship with frontal fog.(3) During the radiation fog and advection radiation fog period of autumn or winter, there always exists an urban moisture island