Power transformer partial discharge signal measurement using Radio Frequency method has advantages over IEC 60270 'apparent charge' measurement method for online monitoring due to greater immunity against external interference. However, the lack of a well-defined calibration relationship between the two methods is main disadvantage of RF method. Simultaneous measurements, using these two methods, carried out on various transformer PD source models, in laboratory, to investigate main parameters that affect the relationship of results of these two methods. In this paper, factors that influence this relationship are classified into three groups: (i) type of PD sources, (ii) transformer windings as PD signal transmitter, and (iii) the inner structure of the transformer. Firstly, the relationship between these two methods has been investigated for some simple types of PD sources. Subsequently, by adding the winding structure model to the void, which is the PD source, its impact on each measurement method and the relationship between them have been studied. The impact of inner structure on the relation between the two methods is compensated through a formula which is proposed as a means for calibration of the RF partial discharge measurement setup. However to compensate for internal insulating system effects more research is required.