Self-regulated writing (SRW) strategies have been used to regulate students' writing in many aspects, especially writing an argumentative essay. Students often feel difficult to write an argumentative essay, so they need to apply SRW strategies to enhance their writing quality. This study was carried out to explore two research objectives; to find out the SRW strategy that is frequently used by EFL undergraduate students from three levels (i.e. high proficiency, middle proficiency, and low proficiency) and to examine the correlation between students' SRW and writing argumentative essay proficiency. This study employed a quantitative approach using two research designs. A descriptive quantitative was applied to find the frequency of use of SRW strategies, and the correlational design was utilized to calculate the correlation between students' SRW and writing argumentative essay proficiency. This current study involved 76 English Department students, and they were required to fill out a Self-Regulated Writing Argumentative Essay Questionnaire (SRWAEQ). The questionnaire consisted of four dimensions (metacognition, cognition, social behavior, and motivational regulation). The students' scores in writing an argumentative essay were used to define their writing proficiency. The collected data were then analyzed using descriptive statistics and inferential statistics in the form of correlational analysis using Pearson Product Moment. For analyzing data, the researcher used the statistical formula of the SPSS 27.0 version. The results of this current study revealed that overall SRW is moderately used by students. Interestingly, all students (students with high, moderate, and low proficiency) prefer to use the social behavior dimension as their self-regulated writing strategies. Also, students from high proficiency and middle proficiency frequently used social behavior and metacognition dimensions for their strategies in writing an argumentative essay. Differently, students with low proficiency only prefer to apply the social behavior dimension as their SRW strategies to write an argumentative essay. The lowest mean of the SRW dimension is motivational regulation strategies. Regarding the correlation, the current study revealed that there was no significant correlation between students’ self-regulated writing strategy and writing proficiency in writing an argumentative essay. It was implied that students’ scores of argumentative essay writing were not influenced by the SRW strategy. Keywords: Argumentative Essay, Self-Regulated Writing, Writing Strategies