Abstract: To realize the high-Tc superconducting (HTS) DC cable system, it is important to study not only high current capacity and low loss of conductor hut also optimum electrical insulation at cryogenic temperature. A model I-ITS DC cable system consists of a HTS conductor, semi-conductor, cooling system and insulating materials. Polypropylene laminated paper (PPLP) has been widely adopted as insulating material for HTS machines. However, the fundamental insulation characteristics of PPLP for the development of HTS DC cable have not been revealed satisfactorily until now. In this paper, we will discuss mainly on the breakdown characteristics of 3 sheets PILP in liquid nitrogen (LN2). The characteristics of the diameter, location of butt-gap, distance between butt-gap length, pressure effect, polarity effect under DC and impulse voltage were studied. Also, the DC polarity reversal breakdown voltage of mini-model cable was measured in LN2 under 0.4 MPa.