The purpose of the study is to investigate the case marking by Thai L1 learners of Korean. Thirty native speakers (NK) and 60 Thai L1 learners participated in the study(30 Intermediate level learners, HL and 30 Beginning level learners, LL). The task was a multiple choice task which was composed of 18 movement verb sentences. The items were divided by case marker alternation (9 alternation constructions and 9 non-alternation constructions). Alternation construction allows replacing ‘ey’ or ‘eyse’ with ‘ul/lul’. Non-alternation construction allows only ‘ul/lul’. The result showed no proficiency variability among the learners. The Learners showed different acceptability judgment from NK by choosing ungrammatical markers in most of the constructions. In non-alternation constructions, NK chose ‘ul/lul’ correctly but the learners failed to choose ‘ul/lul’ and rather chose ‘ey’ or ‘eyse’ erroneously. This observation implicates that more attention and instruction on movement verb construction and case marking alternations should be provided. (Ewha Womans University)