在前人的研究中，已透過音韻規則的角度分析梅縣客語 (Meixian Hakka) 和曼谷客語 (Bangkok Hakka)， 但仍有部分疑問未獲得合理解釋。諸如：部分聲調不會受變調規則(Tone sandhi)的影響、變調的觸發條件以及本調 (Citation tone) 和變調間結構上的關係等。 有鑒於此，本研究透過優選理論 (Optimality theory, OT) 重新分析梅縣客語和曼谷客語。 上述兩個方言有兩種變調的方式 : 同化(assimilation)和異化(dissimilation)。在此基礎上，為了更準確的描述變調現象，本研究在分析上主要採用必要性起伏原則 (obligatory contour principle) 和避免起伏原則 (no-jumping principle)，並應用聯合制約 (Constraint Conjunction approach) 的概念。採取此分析方式的理由在於分析對象的變調具有相當有標(marked)且受限於中心詞(head)右端音節的聲調。此外，本研究也採用一部分比聲調結構性制約(tonal markedness constraint)更高排序的信實性制約(faithfulness constraint)。這些制約會導致部分聲調或變調結構不受變調規則影響，例如：調域 (register) 和聲調的起點 (initial target) 將保留原始樣貌。 研究結果指出，聲調結構性制約和數個排序最高的聲調信實性制約能更準確地呈現梅縣客語和曼谷客語在變調時，輸入值與輸出值之間的對應關係 (input-output correspondence)。在論文結尾，筆者將綜覽本研究並提出未來可繼續延伸的相關議題。 / The grammar of Meixian and Bangkok Hakka tone sandhi has been analyzed from a rule based approach. Nevertheless, there are some questions and details that could not be solved by the analysis, such as the status of tones that do not undergo sandhi, triggers of the tone sandhi, and the structural relation between citation tones and their sandhi counterparts. Thus, the purpose of this study is to re-analyze the tone sandhi in Meixian and Bangkok Hakka under the constraint based framework, Optimality Theory (OT). There are two mechanisms of tonal alternations in the two dialects’ tone sandhi: assimilation, and dissimilation. So in order to capture the tonal alternations, the current analysis applies the concepts of the Obligatory Contour Principle, and the No-Jumping Principle. The constraints generated according to these principles work well with the application of the Constraint Conjunction approach. The conjoined constraints are needed since the tone alternations are highly marked, and depend a lot on the head/right syllable tone. Furthermore, this thesis also posits several faithfulness constraints that rank higher than the tone sandhi markedness constraint. The high ranked faithfulness constraints govern the preservation of several tones from any alternation, and preservation of some structures of the citation tones when they become sandhi tones (i.e. register and initial target). In conclusion, the positing of tone sandhi markedness constraints and undominated identity constraints presents a better input-output correspondence relation of the tone sandhi phenomena in Meixian and Bangkok Hakka. To conclude the thesis, a brief summary of the study and possible further issues are presented.