This research is performed in order to estimate the overall condition of power Transformers, which is considered by 19 test: Site acceptance test, quality oil test and insulation acceptance test. One way is using the Health Index (HI), this technique is useful for maintenance strategy planning. The aim of this research is to represent the new method to assess the conditions of transformer by applying a new developed HI table, which was improved from the conventional method, with Wavelet network and validate by date mining. A case study was used to evaluate the performance of the new develop HI. Therefore, the proposed method can create an efficient preventive maintenance plans for power transformers.