Sensor design is important for electrical capacitance tomography (ECT) to be used for a particular application and for generating high-quality images from the measured data. An ECT sensor with long electrodes is commonly used to obtain stable and reliable measurement signals and to ensure the quality of 2D cross-sectional images, but it neglects the effect of electrode length on the entire 3D images. To analyze the effect of electrode length on the measured signals, sensitivity distribution, quality of reconstructed images, especially distortion of 3D images, ECT sensors with different electrode length, 5, 7, 10, 20 and 30 mm are designed and fabricated. The electrodes are mounted on the internal surface of ECT sensor wall to eliminate the negative effect of the wall. The dual-electrode excitation strategy is chosen to enhance the signal intensity. The results show that the reconstructed 3D images of the measured objects, which can be divided into three parts, are stretched in the axial direction and narrowed in the radial direction because of the axial average effect, and longer electrodes result in more serious problem. Comprehensive analysis of the performance of the ECT sensors with different electrode length, including the measured signals, sensitivity distribution and reconstructed 3D images, concludes that an ECT sensor with electrode length of 10 mm is recommended.