The flexibility of screen-printed silver (Ag) circuits on a polyimide (PI) substrate was investigated under a high temperature and relative humidity (RH). The conductive circuits were constructed on a PI film with a commercial Ag nanopaste via screen printing. The printed patterns were sintered at 200 degrees C for 30 min in a box-type furnace, after which they were placed in a chamber at 85 degrees C/85% RH for various durations: 100, 300, 500, and 1000 h. The Institute for Interconnecting and Packaging Electronic Circuits (IPC) flexural resistance endurance test was conducted to measure the flexibility of the conductive circuits, and the flexibility of the printed patterns was evaluated by detecting the variation of the electrical resistance. The flexibility of the screen-printed conductive circuits decreased as the duration of the 85 degrees C/85% RH test increased. After the 1000 h run of the 85 degrees C/85% RH test, the flexibility of the printed circuits was almost halved compared to that after the 100 h test. To demonstrate the decreased flexibility, the microstructural evolution and partial volume were investigated with a field emission scanning electron microscope (FE-SEM) and a 3D surface profiler, respectively.