The study was aimed to investigate how the use of calculator can enrich the teaching of primary mathematics of Grade Four. Moreover, it helped two participant teachers to use calculators to help their students improve their mathematical thinking. The research question 'How can a teachers use calculators in primary mathematics classroom to improve their students' problem solving and mathematical thinking' was employed to conduct the study in a private school of rural Sindh. A small-scale action research, consisting of pre-intervention, intervention and post-intervention, was conducted to explore and understand the possibilities and challenges of using calculator in primary mathematics classrooms. The study lasted for eight weeks. The data was collected through various qualitative data collection tools such as observation, interview, tape-recording of the discussions and reflections of researcher and participant teachers. The study has identified the factors which facilitate or hinder students' problem solving and thinking process in primary mathematics classroom when calculator is incorporated in primary mathematics. The study reveals the results of the using calculator in teaching primary mathematics, its possible benefits and the expected challenges of incorporating calculator as learning tool in primary mathematics curriculum of Grade Four. This study has significant implication for implementation of the recent National Educational Policy (2009), which puts emphasis on mathematical thinking and problem solving as well as recommends the use of technology in mathematics classrooms.