A large body of research has shown the effectiveness of formative assessment on student learning. Yet research on teachers' implementation of formative assessment is relatively underdeveloped. In addition, there has been a lack of effective instruments for observing teachers' formative assessment practices. This study focuses on the ongoing informal formative assessment and explores the nature of one experienced elementary teacher's formative assessment practices by drawing on a sociocultural perspective and the tool of Formative Assessment Rubrics, Reflection, and Observation Protocol. Primary data sources include classroom observations, interviews, and artifacts. Using inductive and deductive coding approaches, the study found the importance of creating a classroom culture of valuing and foregrounding student ideas when teachers communicate learning targets and elicit students' thinking. In addition, the study sheds light on the teacher's questioning practice with variations during informal FA. The study has implications for future research.