Myrmozercon sternalis n. sp., a new laelapid mite (Acari: Mesostigmata) is described and illustrated based on females collected in the nest of Formica sp. (Hymenoptera: Formicidae) in Karaj region, Iran. The new species differs from almost all other species in the genus by very elongate peritreme, distinctive and horse-shoe like of the sternal shield and also edentate cheliceral digits. The ecological role of Myrmozercon is not clear, but may be parasites of ants