This study investigated the influence of L1 allophonic rule, L1 segment articulation, and gender in Korean learners' production of English /s/ in two different contexts (i.e., before a high front vowel vs. elsewhere) and at three different proficiency levels in English (i.e., low vs. intermediate vs. high). The influence of the Korean allophonic rule realizing /s/ as [voiceless postalveolar fricative] before a high front vowel was not attested. The low and intermediate groups did not exhibit any significant difference between English /s/ before a high front vowel and English /s/ elsewhere regarding centroid frequency (CF). This result illustrates that the low and intermediate groups did not differentiate the articulation of English /s/ in the two contexts regarding its place of articulation. In addition, the high group illustrated a significantly higher CF for English /s/ before a high front vowel than for English /s/ elsewhere, illustrating that the high group pronounced English /s/ before a high front vowel in a more front region of a vocal tract. All the three groups of Korean learners showed significantly higher CF for English /s/ occurring in the context elsewhere in comparison to the native English group, and this may have been due to the influence of an alveo-dental or dental articulation of Korean /s/ (Lee, 1996; Sohn, 1996). Finally, gender effects were attested in the production of English /s/. Male Korean groups at three different proficiency levels in English showed significantly higher CF and PAF (Peak Amplitude Frequency) than the male native English group when pronouncing English /s/ while there was no significant difference among the female groups. It is required to examine whether an alveo-dental or dental articulation of Korean /s/ is more frequently observed in males' speech than in females' in order to say that the result of English /s/ in this study occurred due to the influence of the articulation of Korean /s/.