This paper aims to investigate the optionality of Korean place assimilation. In rule based theory the optionality of place assimilation was attributed to the specific nature of phonological rules. Constraint based theory introduced re-ranking and reversible ranking to explain the optionality. On the basis of markedness theory, I assume the markedness of words and phonological strings is strengthened by optional rules and weakened by obligatory rules. Phonological Complexity (PC, Goldsmith 2002) accurately reflects the changes of markedness and wellformedness by place assimilation. The increase of PC predicts that Korean place assimilation would be prevented optionally to avoid the reinforcement of markedness and illformedness.