This paper discusses why Korean speakers have problems in pronouncing some medial sonorant clusters in English. We argue that the main reasons lie in the sonority sequence requirement difference between the two languages. English does not have any specific sonority sequence preference between the medial sonorant sequences while Korean has a strict requirement between the two sonorants over a syllable boundary. This sonority sequence requirement difference between the two languages acts as an interference for Korean speakers in learning English pronunciation. This barrier for Korean speakers in acquiring correct pronunciation is implemented in a constraint ranking difference in the Optimality Theory, which is not familiar for Korean speakers. Understanding the details of sonorant production mechanisms along with the different constraint ranking will facilitate the learning process of Korean speakers learning English.