[Purpose] This study aimed to compare dynamic balance with respect to completing a single task while not using smartrphone function and completing two task while using different smartphone functions, therby preventing falls or injuries resulting from completion of dual tasks. [Subjects and Methods] The subjects of this study were 36 healthy males and females. The experiment was conducted for five situations: a Star Excursion Balance Test (SEBT) was performed (1) during single-tasking without a smartphone and during dual-tasking with a smartphone, (2) when listening to music using a smartphone, (3) when sending message using a smartphone, (4) when surfing the web using a smartphone, and (5) when playing a game using a smartphone. The condition were the same for all five experimentes. Random selection was done to prevent learing. All experiments were conducted three times, and the averaged values were used for analysis. The SEBT was performed in three directions: anterior, posterolateral, and posterormedial. In consideration the differences in leg length of the subjects, their actual leg length were measured to be used as percentages. Their leg length was measured from the anterior superior iliac spine of the femur to the medial malleolus. [Results] Compared with single task not done using a smartphone, dynamic balance statistically significantly changed for dual tasks done using a smartphone in all three directions. Dynamic balance decreased in all three directions when playing games, sending messages, web surfing, and listening to music. [Conclusion] Completing two tasks using a smartphone reduced cognitive ability, decreasing dynamic balance. Therefore, performing a single task rather than using the diverse functions of a smartphone while walking or working is considered a factor that can prevent falls and injuries.