The present study investigated the influence of processing load upon tuning of spatial attention in the task-relevant (auditory) and task-irrelevant (visual) modalities. Noise bursts were presented via speakers in the free field, and visual stimulus LED was mounted onto each speaker. Processing load was manipulated by alternating auditory targets (one of three possible deviants) randomly in the high load condition, while the auditory target was kept the same in the low load condition. Participants were required to detect auditory targets at the attended location (center or periphery), while ignoring all auditory standard stimuli and all visual stimuli. The N1 attention effect was more negative in the high load condition than in the low load condition. Furthermore, early attention effects were more sharply tuned (steeper ERP gradient) for both the task-relevant auditory and the task-irrelevant visual modalities in the high load condition. These results suggested that both ERP gradients of unimodal and crossmodal spatial attention varied as a function of processing load in the primary modality.