To obtain the high dielectric permittivity and low loss tangent materials for applications in dielectric energy storage capacitors and other high energy density devices,a composite of polyimide (PI) embedded with nanosized giant dielectric permittivity material CaCu3Ti4O12 (CCTO) was fabricated by in-situ polymerization.The distribution of the nanosized particles in the polyimide matrix is homogeneous when the volume fraction of the nanosized CCTO is below 16vol%,but the particles aggregate to form large clusters when the fraction reaches 30vol%,which is confirmed by the SEM (scanning electron microscope) results.The dependences of the dielectric properties of the PI/Nano-CCTO composites on both volume fraction of the fillers and frequency are investigated.The results show that the dielectric permittivities increase with increase in the volume fraction of CCTO at all frequencies (100 Hz- 1 MHz).The maximum of dielectric permittivity is up to 171 at 100Hz with 16vol% of nanosized CCTO at room temperature,which is much higher than the previously reported value (49) of PI/Micro-CCTO composites.Meanwhile,the loss tangent is controlled at 0.46,which is in a relatively low level.Also various theoretical models were employed to rationalize the dielectric behavior of these biphasic composites.The experimental data fits well with the percolation model and the large enhancement observed in the dielectric permittivity of the composites can be explained by the percolation theory.A distinct percolation effect has been found in these composites and they exhibit great increase of the dielectric permittivities near the percolation threshold (fc=0.174) at room temperature.The maximum value of dielectric permittivity emerges with 16vol% of nanosized CCTO,that is when the interparticle distance is close to the diameter of CCTO particles and the interface area becomes extremely large (1.2×107 m2 for the composites with the volume of 1 m3).But with the increase of the volume fraction of the Nano-CCTO particles,the dielectric permittivity of the composite decrease (56 for the composite with 30vol% nanosized CCTO) due to the decrease of the interparticle distance and interface area.And the method of increasing the volume fraction of the ceramic will lead to the lost of the composite’s flexibility.So the high dielectric permittivity of the composite with 16vol% nanosized CCTO is attributed to the well distribution of nanosized CCTO in the polyimide matrix and extremely large interface area between the two phases.The easy processibility,flexibility and excellent dielectric behavior of the PI/Nano-CCTO composites are attractive as potential candidates for the practical applications in energy storage devices,capacitors and electronic devices.