This study investigated the effects of display load (number of positions to be processed) on event-related. potentials (ERPs) during visual search in different mapping conditions. ERPs were recorded from nine adult subjects, who were required to respond if stimuli, which consisted of a horizontal array of five different characters, contained target numbers (consistent-mapping, CM) or target alphabets (varied-mapping, VM). In VM conditions, P300 latency increased and P300 amplitude reduced with increasing display load. Larger display load also resulted in enlargement of search-related negativity. But in contrast, in CM conditions, display load did not affect P300s, nor increase search-related negativity over the whole scalp. The sequential changes of topography of search-related negativity in VM conditions suggests that this negativity may contain three deflections which differed in topography and the time course. A tentative linkage was suggested between the psychological processes involved in the visual search and these deflections as well as the fronto-central N310 and the occipital N190, which were affected by display load both in VM and CM search tasks.