Visual image search on the web is a common behavior that provides people immediate access to the world of published graphical information and the power to search for a desired image. Previous research in the visual search literature primarily focused on the visual search performance (search speed and accuracy), and few studies paid attention to the investment of mental resources in a visual search task. In this study, 40 participants were recruited to perform an image search task. Image set size and time pressure served as two within-subject variables. Performance was evaluated in terms of image search reaction time (RT), the slope of RT × set size function, and search accuracy. Mental workload was assessed based on four physiological indices (heart rate, the power in the 0.1 Hz component of heart rate variability, respiration, and endogenous eye blink) and subjective ratings. The current study found that, regardless of the manipulation of time pressure, the average image search reaction time decreased as the set size increased, producing shallow slopes of the RT × set size functions. There was a positive relationship between task difficulty and mental workload. Higher mental workload was observed when people performed an image search task under time pressure. Moreover, the power in the 0.1 Hz component of heart rate variability was more sensitive to measures relatively low or moderate changes in mental workload. Copyright 2013 by Human Factors and Ergonomics Society, Inc.