Few research studies within the field of mathematics education have focused on the ability to recognize quantities quickly and accurately without counting (i.e. subitizing). The aim of this research was to empirically explore the role of conceptual subitizing activities for enhancing preschool class children's learning of the part-part- whole relations of number. 24 children (aged 6-7) and two teachers participated in the intervention. Due to ethical issues, data were only collected from 18 of the children. The design was a collaborative and iterative intervention, employing a mixed-method approach. Data consisted of pre-and post-test, teacher observation notes and teacher responses to questions about the children's learning. Both the quantitative and qualitative analysis showed that conceptual subitizing activities supported children's knowledge development regarding part-part-whole relations of number. At the group level, the children´s results between pre-and post-test showed an increase of 18.1 percent units and more than half of the children showed conceptual subitizing abilities in a qualitatively more developed way after having participated in the intervention. The result indicated that conceptual subitizing activities might enhance preschool class children's understanding of the part-part-whole relations of number. The results however also elucidated that not all children improved their understanding of the part-part-whole relations of number. Future research should therefore consider individual differences when developing and carrying out interventions.