Knowledge of assessment is a component of pedagogical content knowledge and a teacher's understanding about what to assess in a topic and how to assess it. The aim of this research is to examine the knowledge of assessment of four experienced chemistry teachers in the context of physical and chemical changes. This qualitative research was conducted according to the case study model. The participants were assigned purposively through the criterion sampling technique. Data analysis was performed in the light of content analysis and the data were collected via observation, interview, lesson plan and exam questions. At the end of the research, it was found that the participants' knowledge of assessment were in favor of traditional assessment techniques although the chemistry curriculum inclined them to the alternative assessment techniques. They preferred using questions in open-ended, multiple choice, true-false, multiple choice types. Accordingly, it can be suggested that the experienced chemistry teachers should be more aware of alternative assessment techniques through in-service education programs.