A number of recent studies have shown that familiarity can contribute to associative recognition, specifically when the to-be-paired items are perceived as a single unit; however, whether semantic relationship between two items can help them form a unitization is still heatedly debated, with largely inconsistent results. The present study uses event-related potentials (ERPs) to investigate whether semantic relation can serve as a unitization approach by manipulating levels of unitization (LOU). Results revealed that semantic-related conditions were supported by both familiarity and recollection, whereas unrelated pairs solely by recollection, indicating that semantic relation can encourage unitization. On a larger frame, though, we proposed that the incompatible results among different groups is because of the position of semantic relation on the LOU continuum – just around the threshold to form familiarity, thus unstable enough to produce FN400 in every experiment setting with any particular materials. We further propose that such a threshold position is because of semantic relations helping unitization by having two items share an overlapped representation, which is weaker than other high LOU but beyond-the-threshold relations (e.g. synonyms) that encourage unitization in a stronger manner.