Moisture and ageing in power transformer strongly influence the dielectric properties of oil/paper insulation system. Moisture measurement in oil sample generally gives inconclusive information since oil/paper moisture equilibrium is temperature dependent and takes a long time to be in equilibrium. Direct moisture measurement of paper sample is not practicable for in-service transformers. The measurement and evaluation of the ldquodielectric responserdquo is one possible way of diagnosing a transformer insulation condition. In a recent research project, return voltage measurement has been used for assessing the condition of oil/paper insulation. The return voltage measurement analysis is a non-destructive dielectric testing method for determining the ageing process and moisture content of insulation materials in a transformer. This paper presents a description of return voltage measurement principle explanation and the influencing factors. Furthermore, by analyzing large number of field test data, the criterion of oil-paper ageing is also presented.