Post-metal annealing (PMA) has been adopted to reduce the operation voltages of HfOx-based resistive random access memory (RRAM) devices, especially the forming voltage (VForming). TiN/Ti/HfOx/TiN stack structures were fabricated and annealed via rapid thermal annealing (RTA) and furnace annealing (FA) to investigate the annealing effects. The result of X-ray photoelectron spectroscopy (XPS) analysis indicates that the distribution of oxygen towards the interposing Ti layer increases after the annealing process, which facilitates the formation of conductive filaments in the dielectric layer. As a result, VForming can be decreased from 4.60 to 3.24 and 3.36V via RTA for 30 s at 400 °C and via FA for 60 min at 300 °C, respectively, as compared with that without PMA. However, the VForming of the device annealed via FA for 60min at 400 °C was higher than that at 300 °C. This turn-around phenomenon of the forming voltages of RRAM devices annealed via FA was found. It was attributed to the conversion of the interposing Ti layer into a highly resistive TiO2 film.