This study aims to examine the effects of the collaborative process writing approach on Thai EFL secondary school students' writing skills and investigate their attitudes towards the use of the collaborative process writing approach to developing their writing skills. The participants were sixty-two students studying at a Thai secondary school. This study employed a mixed methods research design. The research involves quantitative and qualitative methods of data collection. Research instruments were (1) the pretest and posttest, (2) questionnaires, and (3) the focus group interview. The results showed that collaborative writing can improve students' writing skills as the scores from the posttest were significantly higher than those of the pretest. Moreover, students had positive attitudes towards collaborative writing as they could share various ideas and help one another develop their written tasks. The results from the interview also showed that the writing process could be a guideline for making a good paragraph in terms of organization, coherence, and accuracy.