Abstract: Organic Electrochemical Transistors (OECTs) have garnered significant research interest in bioelectronics due to their efficient ion-electron transducing capability. However, there were few reports on OECTs with ultra-high cycling stability, which further inhibited their further development and commercialization. Here, to investigate the performance degradation mechanism of OECTs during cycling testing, the differences in cycling stability between planar and vertical OECTs under identical testing conditions were compared, and morphological changes pre- and post-testing were characterized using an optical microscope. Additionally, the performance degradations of vertical OECTs were assessed under different bias conditions. It is indicated that the degradation in the cycling stability of OECTs is attributed to majorly three factors, including repeated doping/de-doping process of ions, bias stress on the source/drain electrodes, and generation of non-capacitive faradaic side-reactions, where all of which contribute to the accelerated performance degradation of OECTs.