This study examined the effects of phonetic similarity between L1 and L2 sounds and L2 experience on the production improvement of L2 English /s/ and /ʃ/ by adult Korean ESL learners. Two production judgment methods were employed. The result shows that a dissimilar L2 sound /s-sh/ was better produced than a similar L2 sound /s/ in terms of listener judgment. Adult L2 learners did not produce similar L2 sounds accurately, even with extensive L2 experience. However, in terms of acoustic-based judgment, ESL learners produced L2 sounds accurately regardless of proficiency level, resulting in a disparity in production performance between two production judgment methods. In assessing the production ability of L2 learners, more than one phonetic-based judgment in addition to listener judgment need to be taken into consideration.