An evaluation study was conducted to investigate the effect of the use of a computer-supported cognitive tool (CT) in the one-to-one classroom setting on student engagement, attainment and perceptions in learning common fractions. Two Primary 4 classes with 68 students participated in this study based on 11 teaching sessions lasting 455 min in total. The students in the experimental group learned the target topic with the use of CT in a one-to-one classroom, while the students in the control group learned the target topic under the traditional teaching approach. The results of a time allocation analysis showed that the use of CT in a one-to-one classroom enhanced student engagement in terms of time-on-task for learning exploration during class time. The results of the post-attainment test indicated that students in the experimental group performed better than those in the control group. Moreover, the questionnaire survey results indicated that students liked to learn the target topic with the use of CT in a one-to-one classroom. This study reveals the potential of the use of CT in a one-to-one classroom to promote classroom-based dialogic interaction in mathematics lessons. It also implies a need for a longitudinal study to investigate suitable pedagogical designs for the use of CT for promoting knowledge transfer in learning challenging mathematics topics. (Contains 13 tables and 3 figures.)