The Caspian (Hyrcanian) lowland forest zone in northern Iran is characterized by small remnant alder forest communities, dominated or subdominated with an Euxino-Hyrcanian element, Alnus glutinosa ssp. barbata. The first floristic inventory of these alder forests in northern Iran is presented. The floristic catalogue is based on the data of 133 phytosociological releves in eight different alder forest sites and on a supplementary floristic inventory of the same sites. Totally, 209 taxa were determined out of which 25 taxa are endemic of Hyrcanian district. Classification based on life form, indicates that the geophytes and phanerophytes make up the largest proportion of the plants in the studied Flora. From the chorological point of view, the largest proportion of the flora belongs to the Euro-Siberian and pluriregional elements. In addition to the total assessment, variation of life forms and phytochoria between different study sites are discussed herewith.There are some similarities and differences in the floristic composition between the study sites based on using similarity indices formula.