Power transformer is one of the most important equipment in the power system. Its operating condition affects the reliability of power supply directly. Therefore, in order to guarantee transformer operation safely and reliably, it is necessary to assess condition of power transformer accurately. Return voltage method based on voltage response measurements is still a new non-intrusive diagnosis method for internal insulation aging of transformer. In this paper the technique and application of return voltage measurement and some results of voltage response measurements of several transformers was introduced. Voltage response measurements were performed on various transformers with different voltage grades, various operating periods, different moisture contents and aging degrees on site. Derived moisture contents from return voltage measurement were compared with the corresponding moisture contents obtained from the analysis of oil samples. Based on on-site experiments and theoretical analysis, the criteria for insulation state of transformer are proposed. Moisture condition of transformer insulation can be determined by using return dominant time constant, and a good correlation between aging degree and the return voltage initial slopes of the aged transformers. Field test performed on several transformers, its interpretation of results are also presented, which proves that return voltage measurements can be used as a reliable tool for evaluating moisture content in transformer insulation.