Mithridates I (B. 195 BC?, D. 138 BC) was the "Great King" of Parthia from ca. 171 BC - 138 BC, succeeding his brother Phraates I. His father was King Phriapatius of Parthia, who died ca. 176 BC). Mithridates I made Parthia into a major political power by expanding the empire to the east, south, and west. During his reign the Parthians took Herat (in 167 BC), Babylonia in (144 BC), Media in (141 BC) and Persia in (139 BC).
Mithridates first expanded Parthia's control eastward by defeating King Eucratides of the Greco-Bactrian Kingdom. This gave Parthia control over Bactria's territory west of the Arius river, the regions of Margiana and Aria (including the city of Herat in 167 BC).
These victories gave Parthia control of the overland trade routes between east and west (the Silk Road and the Persian Royal Road). This control of trade became the foundation of Parthia's wealth and power and was jealously guarded by the Arsacids, who attempted to maintain direct control over the lands through which the major trade routes passed.
In Persia in 139 BC, Mithridates I captured the Seleucid King Demetrius II, and held him captive for 10 years while consolidating his conquests. Demetrius II later married Mithridates I's daughter Rhodogune and had several children with her.
Parthian victories broke the tenuous link with Greeks in the West that had sustained the Hellenistic kingdom of Greco-Bactria, yet Mithridates I actively promoted Hellenism in the areas he controlled and titled himself Philhellene ("friend of the Greeks") on his coins. The coins minted during his reign show the first appearance on Parthian coinage of a Greek-style portrait showing the royal diadem, the standard Greek symbol for kingship. Mithradates I resumed the striking of coins, which had been suspended ever since Arsaces II of Parthia (211–191 BC) had been forced to submit to the Seleucid Antiochus III (223–187 BC) in 206 BC.
His name assigned him to the protection of Mithra and carried the god's authority in some measure.
Mithridates I's son, Phraates (138–128 BC), succeeded him on his death as Great King.