The wide expansion of digital technologies in higher education and the importance of specialised training in teaching writing as a guarantee of school success, have introduced the need to focus on the online professional development for the teaching of writing. This study aims to evaluate the effects of a web-based training for writing instruction on pre-service teachers' knowledge and beliefs under two different delivery methods (i.e. online vs. blended). To meet this goal, 90 pre-service teachers formed the blended group, and 73 pre-service teachers formed the online group. Split-Plot Anovas results indicate that the web-based training seems to be a suitable tool for improving the knowledge regardless of the delivery method and making changes in beliefs due to the training modality. Implications for the initial training of pre-service teachers' in writing instruction are discussed.