The present invention discloses a mobile phone comprising a gravity sensor, a processor, and a memory. The gravity sensor senses inertia data along a specific direction, the processor couples with the gravity sensor and receives a output signal from the gravity sensor, and the memory stores at least one personal information and operates under the processor's control. When either the gravity sensor or the processor detects a vertical free-fall motion, the processor performs a information security process to lock the personal information to become inaccessible.