The present study used steady-state visual evoked potentials (SSVEPs) recorded in parallel to a task-relevant auditory/visual stimulation to study the process of intermodal and crossmodal spatial attention on visual processing. SSVEPs were elicited by task-irrelevant 10/15Hz pattern-reversing checkerboards. The participants were asked to respond to deviant transient stimuli of the attended side in the attended modality only. A phase-locking index (PLI) method was employed to characterize SSVEPs. Both unimodal and crossmodal spatial attention resulted in an increase of PLI values over contralateral occipital brain regions. Intermodal attention effects were observed as an increase of the PLI over the same brain areas when the auditory rather than the visual modality was attended. These findings support recent hypotheses that the phase resetting of the brain activity in early sensory cortices is an essential mechanism of multisensory interaction.