In this paper, we examine the allophones of an underlying segment /l/ in Korean dialects. In particular, we examine how an underlying /l/ sound surfaces in the Korean dialect spoken at Yonbyon, China. To do so, we employ the following processes: First, we perform the phonetic studies on the allophones of an underlying /l/ in the Yonbyon dialect. Secondly, we compare the phonological environments of the allophones of an underlying /l/ in the Yonbyon dialect with the South Korean dialect. Finally, we discuss the phonological implications of the allophones of the underlying /l/ in terms of Feature Geometry and Syllable Contact Law. Based on the phonetic study, we will argue that the distinctive feature [sonorant] should be placed outside the root node and that the flap, an allophone of an underlying /l/, should be understood as an obstruent, not a sonorant.