A declarative knowledge is a crucial component of teacher expertise in education. It describes the utilization of knowledge for noticing highlight in the classroom. However, a teacher may make mistake in the teaching and learning process. We, therefore, want to investigate teacher’s mistake associated with declarative knowledge in mathematical learning. The research is a descriptive study involving a teacher in Primary School. Data were collected through a videotape, an observation, and an interview. The results showed that the teacher demonstrated the mistakes in declaring knowledge (facts and concepts) visually and verbally. The teacher did not realize her mistakes. The teacher also did not predict the classroom situations.