This present study aimed to (1) investigate the effects of project-based writing instruction on writing ability of Thai EFL undergraduate students, (2) examine the effects of project-based writing instruction on critical thinking skills of Thai EFL undergraduate students, and (3) explore the attitudes of Thai EFL undergraduate students towards project-based writing instruction. This study employed a mixed-method research design with a one-group, pre-test-post-test design to collect both quantitative data and qualitative data. The study participants were 24 first-year undergraduate students at Srinakharinwirot University. Five research instruments were created to elicit data including the pre-test and post-test of writing ability and critical thinking skills, the students’ reflective journals, the stimulated recall, the attitude questionnaire, and the semi-structured interview protocol. Data collection took place in the first semester of the academic year 2019, and the collected data were analyzed using descriptive statistics, paired-sample t-test, and content analysis. The findings revealed that after the implementation of project-based writing instruction, students’ writing ability and critical thinking skills increased with statistical significance and students had positive attitudes towards project-based writing instruction. Based on such findings, it could be concluded that project-based writing instruction could be effectively implemented to enhance writing ability and critical thinking skills of Thai EFL Undergraduate students.