本文旨在探討華語焦點變調，並以青年語者為主。本文分別檢驗三種不同的句型，第一類為焦點助動詞和焦點副詞，第二類為平坦結構，第三類則是其它類型的句子。在優選理論的架構下，本文分別提出韻律制約和聲調制約來詮釋華語的焦點變調。筆者透過並存音韻理論(Cophonology Theory)來說明焦點韻律結構的劃分是多個次語法的運作結果。焦點韻律結構的劃分有兩種，主要由(ALIGN-L(F, FP), ALIGN-L(FP, Ft))和(ALIGN-R(F, FP), ALIGN-R(FP, Ft))這兩組可移動的對整制約來決定，當它們在制約排序中移動到不同的位置時會形成不同的韻律結構劃分。本研究發現焦點詞組等同於語調詞組，因此提出另一個對整制約ALIGN-E(FP, IP)來解釋。排序最高的韻律制約則是[FTMIN & NON-FINALITY]，目的在於避免單音節音步出現在句末。至於聲調制約，筆者採用IDENT-BOT來處理三音節音步的變調結果。此外，焦點韻律結構的劃分會造成兩個相鄰的上聲出現在不同的音步之中，OCP-L(ft)可以確保此形式的保留。簡言之，本文藉由優選理論的觀點，以及韻律制約和聲調制約的互動，對華語焦點變調提出了一個整體分析。 / This thesis examines Mandarin focal tone sandhi among Taiwan youngsters. Three types of sentences are under investigation: Type A includes the focused auxiliary verbs and adverbs in a simple sentence, Type B the flat structure, and Type C the other types of structures. Under the framework of Optimality Theory, this thesis proposes prosodic constraints and tonal constraints to account for Mandarin focal tone sandhi. This thesis adopts Cophonology Theory to explain the subgrammars of focal phrasings. There are two types of focal phrasing patterns governed by the reranking of two sets of alignment constraints (ALIGN-L(F, FP), ALIGN-L(FP, Ft)) and (ALIGN-R(F, FP), ALIGN-R(FP, Ft)). The focal phrase pertains to a special kind of intonational phrase. This thesis proposes ALIGN-E(FP, IP) to explain this condition. The undominated constraint, [FTMIN & NON-FINALITY], is proposed to ban a monosyllabic foot in sentence-final positions. IDENT-BOT is adopted to deal with the tri-tonal strings such as (σ(σσ)) and ((σσ)σ) in non-focal and focal readings. In addition, focal phrasing may force adjacent L tones to appear in different feet. OCP-L(ft) is proposed to explain this situation. To conclude, based on the constraint-based theory, and the interaction of prosodic constraints and tonal constraints, this thesis has provided a theoretical generalization of Mandarin focal tone sandhi.