This study investigated the effects of display load on the event-related brain potentials (ERPs) in two different visual letter search tasks. ERPs were recorded from eight normal adult participants, when they were required to judge if a horizontal array of six colored (red or green) alphabets contained target letters. Memory set always consisted of two alphabets. In one task, participants were required to respond to the test stimulus containing a target letter regardless of its color. In another task, a response was made only when the test stimulus containing a target letter was presented in a previously defined color. In both tasks, display load was manipulated by the instruction on the position in an array where a target would appear. In a simple search task, the increase of display load caused the negative shift of ERPs between 300-600 ms post-stimulus. This negativity was dominant over central, parietal, and posterior temporal scalp. In a search task combined with selection by color, display load also resulted in the enhancement of the negativity, but its latency and topography differed from those in a simple search task. Negative shifts appeared about 400 ms after stimulus onset, and had a more frontal and central distribution. Topographic differences of these negativities between two tasks suggest that at least two different systems are involved in visual search tasks, and that the functional organization of these systems could change according to the task requirement. In discussion, a tentative linkage between these ERP changes and two slave systems of working memory was proposed.