御史大夫的地位介於丞相與九卿之間，往往是丞相的候補。釐清西漢政府如何選任御史大夫，可展現具備哪些資歷的官吏才能擔任政府最高階的職位。本文分析西漢御史大夫的任官經歷，勾勒其選任標準形成慣例的過程。西漢前半期（高帝至武帝時期，202-87B.C.）御史大夫常從有軍功者選任，列侯、九卿、郡守、國相皆可升任御史大夫。昭帝以後，選任御史大夫重視官吏的資歷及政績，選治績優異的郡守任九卿、再選九卿為御史大夫的遷轉順序也確立。缺乏軍功、憑任官表現晉升至御史大夫的官吏，在景帝時期開始出現。新的選任標準萌芽於武帝朝，發展至宣帝朝形成慣例。西漢後半期的御史大夫從具備兩種任官資歷的官吏中選用：「地方大吏型」官吏擔任郡守國相或三輔長官有治績，升為九卿、御史大夫；「宮內官型」官吏從未擔任地方大吏， 而是經由高階宮內官升遷至九卿、御史大夫。御史大夫照例從此二型官吏中選用， 然皇帝也可培養自己屬意的官吏，使其達到足以候選御史大夫的資歷。因之，地方大吏型御史大夫又可分成「高第太守」與「補任三輔」兩個子類型；宮內官型御史大夫也可分成「中朝官、理尚書事」與「論議官」兩個子類型。

During the Western Han dynasty, the position of the Imperial Counselor (御史大夫) was second only to that of the Imperial Chancellor (丞相). The Imperial Counselor also often served as a standby Imperial Chancellor. But how were Imperial Counselors chosen? What were their qualifications? To answer these and similar questions, this paper examines the careers of government officials who eventually reached the position of Imperial Counselor. Its main findings are as follows. In the first half of the Western Han (from the reign of Emperor Gaozu to that of Emperor Wu) Imperial Counselors were most frequently officials with a distinguished military career. In addition, Marquises (列侯), Nine Ministers (九卿), Governors (郡守), and Royal Chancellors (王國相) were often promoted to the position of imperial counselor. During the latter half of Western Han (starting with the reign of Emperor Zhao), experience and ability became the main criteria for choosing candidates for Imperial Counselor. A sequence was established whereby one could be promoted from Royal Chancellor or Governor to become one of the Nine Ministers, and from Nine Minister to Imperial Counselor. During the reign of Emperor Jing, officials without an outstanding military career could also be promoted to Imperial Counselor. During the reign of Emperor Wu, new selection criteria emerged, which became firmly established in the reign of Emperor Xuan. From then on, Imperial Counselors were chosen from among two main types of officials. The first type consisted of officials who were promoted to the position of Nine Ministers by virtue of their outstanding performance as Governors, Royal Chancellors or Three Prefects of the Capital Region (三輔 長官). These officials can be classified as ＂governor type＂ (地方大吏型) officials. The second type consists of officials who never served as governors, but were promoted to the position of Nine Minister from high-ranking positions within the imperial palace. These officials can be classified as ＂palace post type＂ (宮內官型) officials.