In Standard Mandarin, a mid vowel agrees in [back] and [round] features with adjacent glides. A mid vowel preceded by a front rounded glide, however, is only partially assimilated, resulting in a front unrounded [e] rather than the expected front rounded [o]. This process of Mid Vowel Assimilation varies across Mandarin dialects depending on the features involved, the direction of feature spreading, and whether or not a mid vowel is fully or partially assimilated. This article proposes a unified constraint-based account of the variation and typology of Mid Vowel Assimilation across Mandarin dialects. Couched within Optimality Theory (Prince and Smolensky 1993) and Feature Class Theory (Padgett 1995, 1996), the proposed analysis demonstrates (i) how the interaction of gradient assimilation-related constraints and markedness constraints on feature cooccurrence avoids marked segment types and produces partial assimilation effects, and (ii) how different rankings of the proposed set of constraints generate the variation pattern across Mandarin dialects. To fully account for the typology of Mandarin Mid Vowel Assimilation and the uneven distribution of different types of Mid Vowel Assimilation, I suggest that (i) some assimilation-related constraints can be categorically assessed as a marked option, and (ii) some constraint rankings on markedness, while not universally fixed, can be designated as universal default rankings.