Assessment of writing skills is a part of teachers’ everyday life. According to previous studies on a similar subject, teachers’ education in assessing writing is limited. The lack of education for teachers may lead to negative consequences for the whole educational system. For this reason, the purpose of this study focused on the analysis of English upper-secondary school teachers’ knowledge, beliefs, and practices in assessing writing in Sweden.  The method for this study was adapted from Crusan et al. (2016) research. An Internet survey was used in order to receive information about teachers’ cognition. Firstly, the survey was published in different Facebook groups, and secondly, it was sent to upper-secondary teachers of English in different counties in Sweden via email. In total, 52 English teachers from upper-secondary schools participated in this study.  The results showed that teachers’ knowledge, beliefs, and practices are co-dependent. Teachers need to be aware of their cognition regarding knowledge, beliefs, and practices. In-service teachers need to receive more training as a form of professional development, while institutions for pre-service teachers need to re-evaluate their educational plans. More research about teachers’ cognition and assessment of writing is needed in Sweden.