Background Entering primary school is an important childhood milestone, marking the beginning of a child's formal education. Yet the change creates a time of vulnerability for the child, the parents and the parent--child relationship. Failure to adjust to the transition may place the family in a psychologically devastating position. The aims of this study were to test the effectiveness of a parental training programme in enhancing the parent--child relationship and decreasing parental stress by reducing harsh parenting in preparing children for the transition to primary school. Methods A randomised controlled trial incorporating a two-group pre-test and repeated post-test was conducted in one of the largest public housing estates in Hong Kong. A total of 142 parents were recruited, with 72 parents randomly assigned to the experimental group and 70 to the control group. Harsh parenting practices, parent--child relationships and parental stress were assessed. Results In comparison to parents in the control group, those in the experimental group engaged in less harsh parenting practices and reported better parent--child relationships. However, parental stress scores did not differ significantly between the two groups. Conclusion This study addressed a gap in the literature by examining the effectiveness of the training programme for enhancing parent--child relationship and decreasing parental stress at the time of a child's transition to primary school. The findings from this study provide empirical evidence of the effectiveness of the parental training programme and highlight the significance of parenting in promoting a smooth transition for children from kindergarten to primary 1. [Copyright of BMC Public Health is the property of BioMed Central Ltd. Full article may be available at the publisher's website: http://dx.doi.org/10.1186/1471-2458-13-1079]