The present study was an investigation of the relationship between the EFL learners’ critical thinking, their frequency, and types of informal fallacy and evidence in argumentative writing. Few studies have been conducted to investigate these issues. To this end, 356-second grade female senior state high school students from four schools in Zanjan were selected through multistage cluster random sampling (MCRS) method and based on Cambridge placement test (2010); 130 students proved to be upper-intermediate and participated in this correlational study. The main data collection stage took place for one month. Then, the informal fallacies based on Johnson's definitions and four types of evidence categorized in Hoeke and Hustinkx were identified and counted within language learners' argumentative writings. The evaluation of the arguments was also conducted based on Walton, Reed, and Macagno. Based on the results achieved from the first research question, there was a significant negative correlation observed between the participants' critical thinking and the frequency of use of informal fallacies in their written argumentation. Based on the results achieved from the second research question, there was a potential and significant correlation between the participants' critical thinking and the frequency of use of informal fallacies.
Keywords: Argumentative Writing, Critical Thinking, Evidence, Informal Fallacy.