This study aimed at comparing two types of IELTS writing task 1 namely, bar charts and tables of Iranian test takers in terms of four IELTS writing assessment criteria including task achievement, coherence and cohesion, lexical resource, and grammar range and accuracy. The study also sought to identify test takers' strengths and weaknesses in the writing task 1 with respect to all four areas. To this end, forty-five Iranian IELTS test takers aged 19 to 42 were involved in the study. In the beginning, the participants were assigned to perform a bar chart IELTS writing task 1. Within 10 days, the same test takers were presented with a table description task. Their writing performances were then rated based on the IELTS writing rubric. The results indicated that the candidates' writing performance was significantly higher in bar chart description tasks than table-based writing tasks. To provide a detailed picture of the contribution of each criterion to the total score variance, we performed a multi-linear regression analysis, the results of which manifested that the candidates were more adept at accomplishing task achievement, and grammar range and accuracy in table and bar chart writing tasks, respectively. The findings also indicated that they did not do well in terms of coherence and cohesion performing table writing. Nor did they perform effectively on the lexical resource in bar chart writing. The results of the study carry crucial pedagogical implications for teachers, IELTS trainers, researchers, and syllabus designers.