The assessment of health state of power transformer is a key step to realize Condition-based Maintenance of transformer. Research on health state assessment of transformer has very important actual significance. Transformer can be recognized as a typical grey system. The assessment of its healthy state using grey clustering decision-making method is put forward. The grey classes of transformer are given, the whitenization weight function of grey classes is established, the information involved in assessing the state of transformer is analyzed, and finally steps of state assessment of transformer using grey clustering decision-making are presented. The analysis of actual example demonstrates that the method has provided a new method to assess the health state of transformer.