Despite the one-to-one correspondence between the letters and the sounds in Hangul, pronunciation varies depending upon the context of the phonological rule application. This is where the difficulty for the learners of Korean as a foreign language (KFL) arises. The current study investigates the efficacy of explicit and implicit instruction on reading aloud written Korean, focusing specifically on four phonological variation rules: resyllabification, obstruent nasal assimilation, h-deletion, and aspiration. The results of a classroom-based intervention study involving 74 adult KFL Korean learners showed that explicit instruction was more effective than implicit learning for applying the rule of obstruent nasal assimilation and aspiration, which requires relatively complicated steps of cognitive operation. For applying the resyllabification and h-deletion rules, both explicit and implicit instruction methods were found to be equally effective. This result may be attributable to the combined influence of practice time allocated to each group, the relationship between orthography and phonological rules, and the frequency of the phonological rules. Implications for teaching are discussed in relation to the experimental results.