This study examines the influence of illuminance variation on visual perception and mood in a small office. Field measurements and annoyance tests were performed in a full-scale mock-up office space. Subjects conducted paper-based and computer-based reading tasks when six instantaneous fluctuation ranges were given under 500 lx and 650 lx base level conditions. Results indicate that equal amounts of instantaneous illuminance fluctuation could influence visual perception differently under these two base level conditions. Visual annoyance under the 500 lx base level was more severe than that under the 650 lx base level. The acceptable illuminance fluctuation ranges that did not cause visual annoyance under 500 lx base level conditions were 141.3 lx for paper-based tasks and 187.3 lx for computer-based tasks. Under 650 lx base level conditions, the acceptable ranges were 200.3 lx and 252.4 lx for paper-based and computer-based tasks respectively. The mean mood and perception responses for visual thresholds under the 650 lx base level showed more positive feelings than those under the 500 lx base level. Multiple linear predictive models showed that feelings of visual sensitiveness to illuminance variation, visual distraction, and stimulation were significant contributors to visual annoyance under fluctuating illuminance conditions.