Phraates I of Parthia , ruler of the Parthian Empire from ca. 168–165 BCE, perhaps succeeded his father Phriapatius (185–170 BCE) on the throne. However, recent evidence from Nisa suggests that a great-grandson of Arsaces I reigned briefly after Priapatius' death in 170. In this reconstruction, this previously unknown "Arsaces IV" reigned ca. 170-168 BCE and was then succeeded by Phraates I (this of course would make Phraates Arsaces V). Phraates I died relatively young, and appointed as his successor not one of his sons, but his brother Mithridates I (165–132 BCE).
He subdued the Amardians (lat. Amardis), mountaineers occupying eastern portion of the Elburz range, south of the Caspian Sea.