This study investigates how Korean high school students with different levels of English writing are able to utilize their own self-regulated learning strategies and a sense of self-efficacy in writing classrooms. A total of 128 students took part in this study and they were classified into high-, intermediate-, and low-proficiency groups. Four instruments were used in the study: a background questionnaire, a English writing test, the Writing Strategies for Self-Regulation Learning Questionnaire (WSSRLQ,) and the Second Language Writer Self-Efficacy Scale (L2WSS). When looking at the results from the WSSRLQ, the results show that the high-level writers significantly used text processing and goal-oriented monitoring and evaluating more often than those in the intermediate- and low-level groups; the high-level group also used idea planning, interest enhancement, and emotional control significantly more often than the low-level group. In regards to the L2WSS, the high-level learners demonstrated greater linguistic self-efficacy, self-regulatory efficacy, and performance self-efficacy. In addition, there were positive correlations among self-regulation, self-efficacy, and English writing competence. Based on the results, suggestions are given for teachers, so they can better help their students improve writing skills.