This study investigates the extent to which the four variables of initial age of L2 learning, length and frequency of private English education, and percentage use of L2 have influence on the discrimination of the three English vowel pairs /iy-ɪ, uw-ʊ, ɛ-æ/, which tend to pose problems for Korean children learning English in an instructed minimal input context. In this study, a questionnaire inquiring of the four variables and a discrimination task of the three vowel contrasts were conducted to forty-nine 4th graders in elementary school. The four variables predictive of the attainment of L2 pronunciation in naturalistic SL settings were found not to have significant influence on the perception of the six vowels by the participants. Based on this result, the importance for the explicit teaching of L2 pronunciation in an instructed FL context and the need for L2 teachers’ education in the fields of phonetics, phonology, and L2 pronunciation teaching were emphasized.