本文以優選理論的觀點分析美濃客語的陰平變調。討論可分為兩部分，首先，針對於變調的範疇，文章從句法與音韻的介面關係探討陰平變調的範疇，結果顯示陰平變調是以語調詞組(Intonational Phrase)作為變調的範疇，而這也為韻律結構的存在提供了證據。語調詞組的形成與感知單位(Sense Unit Condition)和重組(Restructuring)有密切的關係。另外，也探討了語調詞組中的特殊結構，發現語料支持Nespor & Vogel (1986)的假設，即特殊結構可以先形成一個獨立的語調詞組。 探討完變調範疇之後，將以優選理論進行分析。本文提出兩組制約:聲調制約和韻律制約。聲調制約著重在解釋變調的運作，而韻律制約則負責解釋變調範疇的劃分，這兩組制約不僅能解釋一般句子的變調，亦能對特殊結構的變調現象予以充分的說明。針對於變讀的情形，本文以並存音韻理論(Cophonology Theory)來作解釋，藉由*IP和ALIGN-R (Non-adjunct XP, IP)兩個制約的重新排序，得到變調與不變調兩種讀法的範疇的劃分。 / This thesis is aimed at analyzing Yinping tone sandhi in Meinong Hakka from the perspective of Optimality Theory. The discussion is divided into two parts. First, this thesis explores the tonal domain in terms of the interface of syntax and phonology. The analysis indicates that the tonal domain is founded on an intonational phrase. The formation of an intonational phrase is based on the Sense Unit Condition (Selkirk, 1984) and is closely related to restructuring (Nespor and Vogel, 1986). The special constructions of IPs are also investigated. The data supports the hypothesis proposed by Nespor and Vogel (1986), namely, that these special constructions have the privilege to form an isolated intonational phrase. Secondly, Optimality Theory is adopted to analyze the data after exploring the tonal domain. Two set of constraints are proposed in this thesis. One is that of tonal constraints, which are mainly concerned with the processing of tone sandhi. The other is that of prosodic constraints, which are responsible for explaining the delimitation of the tonal domains of the data. The two sets of constraints can be used to account for the operation of the sandhi phenomena in ordinary sentences and also in special constructions. The alternative readings are explained by means of the re-ranking of *IP and ALIGN-R (Non-adjunct XP, IP) from the perspective of Cophonology Theory.