This study takes another look at a moot problem in the analysis of Korean sub-compounds, where the second component nouns are y-initial nouns, with n/Ø alternation between two nouns. There have been two opposing approaches to this alternation: one for an /n/-insertion solution and the other for an /n/-deletion solution. However, each approach has left behind some data unaccounted for. Drawing on what is observed in Han (1994), we present another analysis to this n/⌽ alternation in a constraint-based framework of Optimality Theory. Our suggestion is that an underlying /n/ should be posited for some nouns, but not for all nouns uniformly. We argue that regardless of native Korean or Sino-Korean combinations, /n/-Insertion applies to y-initial components, only when the first components are prosodic stems. Some exceptions result from the weakening of a stem boundary, which is related with variables such as the number of syllables and the combining force or semantic relationship between the two components, and frequency effects of sub-compounds. The boundary status of the first components is shown to play a pivotal role in deciding that of the following components, backed by the condition that sisters in prosodic structure be of the same type.