Safe driving requires the driver to maintain situation awareness of dynamic traffic information, including surrounding vehicles' spatial information and other properties. Previous research has shown that performing mental secondary tasks drastically changes drivers' visual scanning behaviors (Recarte & Nunes, 2000). But it remains unclear whether the modified visual scanning pattern results in deterioration in drivers' situation awareness of surrounding traffic. More specifically whether different aspects of drivers' situation awareness would be affected by secondary tasks differently, or whether different types of secondary task would interact with the different aspects of the situation awareness. This study employed a change detection paradigm (Zheng, McConkie, & Tai, 2003) to investigate these questions in a simulated dynamic driving environment. The results showed the general negative effect of secondary tasks on the driver's situation awareness, with the spatial-imagery task producing greater interference than the verbal task. Moreover, both secondary tasks interfere more with the detection of the vehicle location displacements than with the detection of the vehicle color or identity changes.