Electrical and mechanical properties of polymer sheet at cryogenic temperature have been investigated. Tensile stress(and strain at break) in liquid nitrogen(77K) of 79.7MPa(l.2%) and 117.4MPa(2.05%) are evaluated for films of Polypropylene (PP) and Kapton, respectively. Dielectric loss tangent(tan .delta.) of PP and Kapton films is almost independent of the frequency and tensile stress. Also, field strength of PP film at 77K decreases with increasing the tensile stress.