In modern Persian the preposition "az" signifies the concepts instrument, method, cause, nature, possession, part-whole relationship, source, path, separation, comparison and topic. "az" is the remainder of the preposition hača in Avesta and sačā in Sanskrit. This preposition has been taken from the root "sac/hak" meaning accompaniment and has been used in Middle Persian and Parthian as "az" and "až". In Sanskrit sacā has been used to convey accompaniment. This preposition has undergone two levels of grammaticalization over time: in the first step noun has been converted to preposition to mean “accompanied with” and other meanings derived from it, and in the second step it has been used to convey other meanings such as source and other related senses. The present paper is an attempt to investigate the polysemy of "az" based on the conceptual transfer model.