This study aims to develop an understanding about how formative assessment can be used to develop students' conceptual understanding in the selected concepts of science at Grade 6. The study utilized the action research method within qualitative paradigm. It was conducted in an English medium Girls High school in Karachi. The school had been using formative assessment rather than summative assessment to assess students' learning from grade 3-6. Hence, it was decided to explore how formative assessment can be used to develop students' conceptual understanding in this school as it would develop my understanding about usage of formative assessment. It would also support the school's effort to implement FA in this grade in science. Data was mainly collected from observations and teaching by using various formative assessment tools such as peer group assessment, Think-Pair-Share activity and classroom questioning. Findings of the study demonstrated that it is vital for a teacher to diagnose the prior understanding of the students regarding any science concept to ensure their conceptual understanding. It also revealed that group-work, peer-assessment and Think- Pair-Share activities are more likely to help students in developing their conceptual understanding. It further showed that teachers need to analyze the assessment data on an ongoing basis to gain insight into the needs of students and to modify teaching instructions accordingly. It also indicated that timely feedback given by teachers and peers supported students' learning and developed their conceptual understanding. However, strict time frameworks of schools and heavy syllabus hinder the effective implementation of formative assessment in real classroom context. This study concludes by recommending that for effective implementation of formative assessment, teachers needs support in terms of time and space from the school management and they must be encouraged to analyze the assessment data to inform their teaching practice. Therefore, the study ca11s for the change in teaching and assessment practices in schools. Key words: Formative assessment, conceptual understanding, summative assessment.