In respect of manual control car navigation system, control safety of in-vehicle device is assured by restricting the driver's complex operations such as setting or revising destination by operating cursor switch while driving. Because the driver's workload when using voice-controlled system is small, voice operation is adopted as an effective operation method to reduce the driver distraction. In this research paper, voice control, a method of reducing driver’s distraction, is investigated and researched by testing under different driving and control conditions to determine the influence of the loads.