This study intends to examine how the major rules of Mandarin third tone sandhi function as well as what their structures and patterns are in various situations. An investigation of this attempt finds that the third tone sandhi in Chinese Mandarin is a kind of phonological dissimilation process in which the third tone changes across word boundaries, depending on what kind of morpheme follows this third tone. Two major rules govern the tone sandhi of Mandarin third tone. When a third-tone syllable is followed by any tone, other than another third tone, it changes to half a third tone. When a third tone is followed by another third tone, the first third tone changes to a second one. Larger combinations of the third tone seems to be governed by the same rules. In addition, this study claims that Mandarin third tone sandhi is also determined by the syntactic structure of the sentence as well as semantic or attitudinal factors.