Reflection to maintain classroom momentum, to overcome unwanted practices e.g. 'shyness', 'shivering' and 'repetition of words' during the process of teaching, led student teachers to shift their attention towards acquisition of professional skills during practicum. This paper examines the role of Reflection-in-Action to acquire professional skills during practicum. Quantitative data was collected from 800 student teachers through a questionnaire. Likewise, 10% of the total sample was observed to collect qualitative evidences. Descriptive analysis of the data was made to see the tendencies of the data and then variables were computed to test the hypotheses. Reflection of student teachers over students' discourteous attitudes, 'student teachers' own unwanted practices (shyness, shivering and repetition of words), class room momentum, class room discipline and reflection over hesitation to respond to students' questions nearly increased 49% chances for student teachers to smoothly complete their lesson plans. A significant relationship exists between Reflection-in-Action and smooth progression of the lesson plan. Reflection-in-action significantly contributes to wards professional development of student teachers.