This study was purposed to discuss an educational concept of listening potential in ESL/EFL contexts as a counterpart of the reading potential concept in L1 contexts. As a preliminary condition for the concept of listening potential, this study investigated a relationship between reading and listening proficiency of ESL/EFL learners, focusing on roles of grammar knowledge in reading and listening comprehension. Reading and listening comprehension and grammar knowledge test scores of 231 Korean college students studying at a university in the U.S. were analyzed for this study. This study found that there was a positive correlation between reading proficiency and listening proficiency of Korean ESL college students. Grammar knowledge played significant roles accounting for reading and listening comprehension. Furthermore, this study found a difference between the reading proficiency and the listening proficiency across the stages of language development. However, results of this study did not empirically verify the concept of listening potential.