The resistive random-access memory (RRAM) is a promising candidate for next-generation nonvolatile memory systems owing to its simple structure and low power consumption. In this study, the oxygen-vacancy-based filaments in HfOx-based RRAM were controlled using rapid thermal annealing (RTA) to achieve thin, defined filaments through grain boundary engineering. With increasing RTA temperature, the grain size of the HfOx layer increased, and the grains merged with other grains to form simplified grain boundaries, along with oxygen vacancies, facilitating the formation of thin filaments. To investigate the formation of conductive filaments within the RRAM devices, a bilayer structure composed of annealed and unannealed HfOx layers was fabricated. When the RTA temperature was raised from 300 to 500 °C, the RRAM exhibited typical bipolar resistive switching (BRS) characteristics. Conversely, when the RTA temperature was increased to 600 °C, differences in the process of filament formation from the bottom and top of the HfOx layer led to the emergence of complementary resistive switching (CRS) characteristics. CRS has superior potential for memory applications compared with BRS owing to the ability of the former to suppress the sneak-path issue. Notably, CRS characteristics were developed in layers deposited on the same materials and equipment rather than through a bilayer of different materials or by the addition of a metal layer in the middle. The present results suggest that filaments formed through RTA-induced grain boundary engineering have potential advantages over those from the conventional RRAM.