Disclosed herein are a lane-change assistance system of a vehicle and a lane-change assistance method thereof. The lane change assistance system includes a wheel speed sensor to detect a wheel speed of the vehicle, a steering angle sensor to detect a steering angle of the vehicle, and a micro controller unit to calculate, using the wheel speed and the steering angle, a desired yaw rate value required to allow the vehicle to return to an original lane when attempting to move the vehicle to an adjacent lane, but another vehicle in the adjacent lane threatens the corresponding vehicle, to compare the desired yaw rate value with a preset threshold value, and to assist a lane change maneuver of the vehicle based on the comparison result.