Summary form only given. We have synthesized highly stretchable and highly conducting polyacetylene films by Intrinsic Non-Solvent (INS) polymerization method and investigated the mechanical and electrical properties of the films, in order to examine the feasibility of the INS method as an advanced solvent-free polymerization method. The mechanical strengths of the films are 40 GPa in Young's modulus and 2.1 GPa in tensile strength. The electrical conductivities upon iodine doping are 2.2 - 4.3 x 10/sub 4/ S/cm, which are enhanced by 6 - 9 fold mechanical stretching. The results imply that the present films are more easily stretchable but less breakable than the films prepared by the solvent evacuation method (Young's modulus of 100 GPa and tensile strength of 0.9 GPa), leading to a remark that the INS method produces high quality polyacetylene films whose, mechanical and electrical properties are comparable and/or superior to those by the solvent evacuation method.