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Twenty-nine acritarch species belonging to fourteen genera are documented from the Early Ordovician sediments of the Central Alborz Range in the Hassanakdar area, northern Iran. The encountered acritarch taxa consist of Acanthodiacrodium sputum, Acanthodiacrodium bicoronatum, Acanthodiacrodium complanatum, Acanthodiacrodium zonaconstrictum, Acanthodiacrodium seratimum, Acanthodiacrodium vavrdovae, Acanthodiacrodium staleness, Cymatiogalea diversita, Cymatiogalea cylindrata, Cymatiogalea cristata, Cymatiogalea bellicosa, Cymatiogalea membranispina, Leiosphaeridia Sp., Vulcanisphaera africana, Vulcanisphaera nebulosa, Goniosphaeridium dentatum, Goniosphaeridium sufflatum, Goniosphaeridium tener, Arbusculidium filamentosum, Arbusculidium rommelaeriei, priscotheca tumida, priscotheca raia, Dasydiacrodium polarum, Lophosphaeridium sp., Saharidia downiei, Dactylofusa squama, Coryphidium elegans, Athabascaella rossii and Athabascaella penika.
The above-mentioned acritarch taxa have been arranged in three ascending stratigraphic assemblage zones. Based on these three assemblage zones, the Lower Ordovician (Tremadocian-Arenigian) is suggested for the Ordovician sediments (member five of Mila Formation) of the Hassanakdar area. The encountered acritarch taxa of the studied area were made comparision with those which have been recorded from other parts of the World, especially Southern Europe, nothern Africa, south-west China, North America and The Zagros basin of Iran. This comparison indicates a presence of fairly homogeneous acritarch assemblages which extend from east Newfoundland through Mediterranean to northern and southern Iran.