The synaptic linearity of resistive random-access memory (RRAM) based on TiO x /HfO2 improved by inserting an ultrathin Al2O3 layer is investigated. A gradual bipolar switching with a positive set and a negative reset is observed for devices with an Al2O3 layer after an electroforming process. The devices with a 1 nm Al2O3 layer exhibit acceptable reliability with >400 cycles DC endurance with no decrement of the on/off ratio after 104 sec. A remarkable enhancement in the synaptic linearity of potentiation 2.15 and depression 1.52 is achieved in this device. The conduction mechanisms at different current regions of the optimized device are studied. The presence of the Al2O3 layer is confirmed by x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy analyses. About 42% of the oxygen vacancy concentration calculated from the XPS spectra is responsible for the synaptic properties. This synaptic RRAM structure is suitable for upcoming neuromorphic computing devices.