Despite the importance of Continuous Professional Development in the EFL context, a limited research has been done on EFL teachers' collaborative reflection on teaching. This study aims to investigate the effectiveness of teachers' reflection that is fostered and enhanced by the collaborative written exploration of practice using an online-shared teacher journal in a Saudi tertiary context. To achieve this, the study adopted three theoretical foundations: the characteristics of the online teacher professional development, Vygotsky's zone of proximal development and the constructivist theory of learning. The level of reflection in the teachers' journal is analysed by using the discourse semantics approach, namely, systemic functional linguistics. Besides, a semi-structured interview is used to collect the qualitative data. They strongly suggest that maintaining an online-shared teachers' journal will support and augment teachers' reflection, enhance knowledge and improve teaching performance.