Various specimens of nylon 9 and nylon 11 were prepared, and their crystalline structures, the temperature and frequency dependences of electric properties were investigated. Dielectric constants (at 10 Hz).of various specimens were about 2.5—3 in the temperature range from −160°C to room temperature, and they began to increase near Tg and reached above 30 at about 100°C. They showed three dielectric loss peaks due to two types of local mode motion and the glass transition. Piezoelectric behavior of the α form specimen obtained by slow cooling of melts and that of the γ form specimen obtained by casting from its trifluoroacetic acid solution were almost the same, but a mixture of α and γ forms obtained by quenching of the melts into liquid nitrogen had a larger piezoelectric constant than the above two. From these results it may be considered that the lower regularity in the dipole orientation within the mixture makes it relatively easier to rearrange the dipoles in parallel to the electric field in the poling process. Nylon 9 had a larger piezoelectric constant than nylon 11, which may be due to the higher dipole density of nylon 9. The piezoelectric stress constant of the stretched film was larger than the unstretched one, but the piezoelectric strain constant of both were similar. The largest piezoelectric stress constant of 2.1 mCm−2 (at 25°C) was obtained from a stretched nylon 9 specimen and the largest piezoelectric strain constant of 1.1 pCN−1 (at 25°C) was obtained for nylon 9 specimen which was a mixture of α and γ forms.