|Millennium:||1st millennium BC|
|Centuries:||3rd century BC – 2nd century BC – 1st century BC|
|Decades:||160s BC 150s BC 140s BC – 130s BC – 120s BC 110s BC 100s BC|
|Years:||141 BC 140 BC 139 BC – 138 BC – 137 BC 136 BC 135 BC|
|138 BC by topic|
|Gregorian calendar||138 BC|
|Ab urbe condita||616|
|Ancient Egypt era||XXXIII dynasty, 186|
|- Pharaoh||Ptolemy VIII Physcon, 8|
|Ancient Greek era||160th Olympiad, year 3|
|Chinese calendar||壬寅年 (Water Tiger)
2559 or 2499
— to —
癸卯年 (Water Rabbit)
2560 or 2500
|Coptic calendar||−421 – −420|
|Ethiopian calendar||−145 – −144|
|- Vikram Samvat||−81 – −80|
|- Shaka Samvat||N/A|
|- Kali Yuga||2964–2965|
|Iranian calendar||759 BP – 758 BP|
|Islamic calendar||782 BH – 781 BH|
|Minguo calendar||2049 before ROC
|Seleucid era||174/175 AG|
|Thai solar calendar||405–406|
|Wikimedia Commons has media related to 138 BC.|
Year 138 BC was a year of the pre-Julian Roman calendar. At the time it was known as the Year of the Consulship of Serapio and Callaicus (or, less frequently, year 616 Ab urbe condita). The denomination 138 BC for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
- Zhang Qian begins his explorations in central Asia for Chinese emperor Han Wu Di.
- The first Chinese diplomatic mission to the Ferghana valley, led by Chang Chien, is sent.
- Han Dynasty China intervenes in a war between the Minyue and Eastern Ou during its expansion southward.
Arts and sciences
- Hymn to Apollo is written and inscribed on stone in Delphi; it is the earliest surviving notated music, in a substantial and legible fragment, in the western world.
- Diodotus Tryphon (suicide)
- Mithridates I, King of Parthia (b. c. 195 BC)
- Attalus II Philadelphus, King of Pergamon (b. 220 BC)