傳統上，對一個國家憲政體制的分類與運作主要依據其憲法條文的規定來判斷權力之間的分立與制衡，但是隨著民主的發展，政黨的產生實際上已改變了原本憲法中對於權力間分立與制衡的規定，行政與立法間的緊密與區格往往是由正黨的性質，與政黨體制的不同來決定，同時，憲政體制的不同也會對政黨的組織與政黨的體系產生不同程度的影響，因此如果想要對一個國家的政制結構有深刻的瞭解，對於憲政體制與政黨政治間互動及影響的探討是必須而且重要的。 然而，政黨體系的產生與維持，選舉制度的不同又是一個重的變項，不同的選舉制度往往也造就了不同的正黨體系同時對政黨組織的發展產生影響，因此，憲政體制、政黨政治與選舉制度三者間的關係其實是複雜而且相關的，本文的焦點即是對於這樣的三角關係加以探討，藉由對美、英、法三個不同國家間不同的憲政體制、政黨政治與選舉制度的討論期望能幫助我們更瞭解三者間的關係與三國實際的政治運作，同時也對我國現行的制度作一個比較，並對未來的展望加以討論。

This study analyzes the triangular relationships between Constitutional System, Party Politics and Electoral Systems. The internal structural charactyeritics of major parties and the types of party systems exert a profound influence upon the operation of constitutional systems in general and the separation of powers in particular. And the electoral systems, in turn, have important effects on organizational structure of parties and changes in party systems. This paper reexamines the Duverger’s law and his hypothesis, Duverger did not differentiate different types among multi-party system, thus he could not distinguish different political consequences between the second-ballot system and proportional representation system. This paper point out the tendency relationships between second-ballot systems and moderate multi-party system and proportional representation and polarized multi-party system. This paper avoids causal assertions, the triangular relationships are argued only as probabilistic associations. Causal analysis of such complex political phenomena requires a much fuller examination of other variables which is beyond the scope of this paper.