西晉以後司馬氏的發展之所以導致門閥士族的快速成長及皇權衰微之結果，其肇因在西晉政權建立前的司馬懿至司馬昭時代大致已佈下了。司馬防、司馬懿至司馬昭，正是司馬氏由地方儒學家族，走到統治者的過程。司馬懿對於其個人發展、家族延續與政治突破的思考，都隨著其在曹魏政權中權勢的沈浮，而不斷的改變。司馬懿在進入仕途後的第四十九年，以遭受曹爽等集團人士的壓迫，出手一擊，用極嚴峻的殺戮，對付了異己勢力。至此司馬懿的注意力已集中在權力的攫得，未入仕之前，對天下大局的抱負與使命感，早已隨著政治的鬥爭，逐漸消失無形了。由此也就可以解釋，在漢魏之際許多原與曹氏結合的儒生，在曹操漸露奪權之相時，相繼離操而去，但司馬氏一家反而與曹氏的關係日漸拉近，變成一體的原因。 本文在過去的研究基礎上，嘗試從司馬氏家族個體在政治上的浮沈，導致司馬懿等人，在發展過程中，對曹魏政權、對司馬家族及對自身的定位都不斷地調整，由此來解釋司馬懿、司馬師與司馬昭，在掌權後，先有努力作為，最後卻都走上大殺戮的道途的結果。

The development of Sima Clan during Western Jin Dynasty has resulted in the fast growing of powerful families and the reduction of authority of the empire. However, this situation has already been establishing during the pre-Western Jin Dynasty Era by Sima Yi and Sima Zhao. Sima Fung, Sima Yi, and Sima Zhao, the three generations represented the course of transition from the Confucianists to political rulers. It was more significant in Sima Yi whose view of world has changed drastically from peace to power struggle and eventually to oppression of oppositions. This paper studied the political ups and downs of the Sima Clan that led to the constant adjustment of their view of the world. This explained why Sima Yi, Sima Shi, and Sima Zhao were leaders with aspiration to begin with and ended up ruthless rulers.