本論文以優選理論分析六家饒平客語和寧都田頭客語雙音節詞之連讀變調，並藉由聲調內部結構說明變調的動機。在六家饒平客語方面，主要是呈現位置變調(positional tone sandhi)，較有標(more marked)的聲調位於前字時發生變調。另外，此方言的陽平和陽去因歷史演變，兩者的本調(citation tone)同為HH，但兩者因不同的變調形式而產生不同的變調(sandhi tone)。陽去的變調形式為位置變調，而陽平的變調形式為環境變調(contextual tone sandhi)。本文採用聯合制約(Local Constraint Conjunction)捕捉環境變調制約運作的環境。另外，運用「詞素特定音韻」(morpheme-specific phonology)標記聯合制約，解釋陽平和陽去不同的變調規則。在寧都田頭客語方面，變調受詞法結構影響，且兩個音節皆有可能發生變調。本文利用標記制約理論(indexed constraints approach)解釋不同結構的變調情形。另藉由位置信實制約不同的排序，嘗試說明聲調的保留屬於類型差異(typological differences)。而此方言的變調類型包含位置變調和環境變調，同樣要求較有標的聲調改變，並使用聯合制約說明在環境變調的情況下制約運作的環境。 / This thesis investigates the tone sandhi in Liujia Raoping Hakka and Ningdu Tiantou Hakka under the framework of Optimality Theory. The internal structure of the tone reveals the motivation and the mechanism of tone sandhi. In terms of the tone sandhi in Liujia Raoping Hakka, the universal tonal markedness tendency could be regarded as the motivation triggering the positional tone sandhi in the left syllable. Moreover, the morpheme-specific phonology is adapted to account for the tone sandhi of historical merged tones, Yangping and Yangqu. Yangping and Yangqu are both high level tones; however, they display different tone sandhi patterns. Yangping displays the contextual tone sandhi whereas Yangqu displays the positional tone sandhi. Local conjunction constraints are posited to restrict markedness constraints to specific contexts in order to account for the mechanism of contextual tone sandhi. On the other hand, in terms of the tone sandhi in Ningdu Tiantou Hakka, the tone sandhi is construction sensitive and takes place in both syllables. The indexed constraint approach is adopted to explain the tone sandhi in different constructions. In addition, the preservation of tone is argued to be the typological difference according to different rankings of the positional faithfulness constraints. Finally, the tonal markedness tendency motivates the positional tone sandhi and the conjoined constraints are posited to govern the contextual tone sandhi in this dialect.