As smartphone usage increases, safety concerns have arisen. Previous research suggested cognitive impairments while using mobile devices in walking conditions. Mobile user interfaces that are designed in ways not to require users' full attention may mitigate the safety concerns. Primary focus of this research was on the perception process during visual search rather than the physical target selection by finger tapping, which most previous research focused on. The effects of object size, contrast, and target location on mobile devices while walking and standing were examined. A serial visual search using "T" and "L" shapes on a mobile device was conducted, which controlled for the physical target selection involvement. The results showed that walking, bigger object size, and the target position in the outer area of the mobile device display slowed the visual search reaction time. This suggests mobile interface improvement possibilities by proper object sizing and placement.