Disclosed here are an active cruise control system in vehicle and method thereof. The active cruise control system in a vehicle includes: a vehicle sensing portion which measures driving speed, acceleration and a steering angle of the vehicle; a preceding vehicle sensing portion which senses a preceding vehicle which is driving in front of the vehicle; and a controller which performs lateral control based on the driving speed, the acceleration and the steering angle which have been measured and performs acceleration and deceleration control to keep a predetermined distance from the preceding vehicle when the measured driving speed which is a preset critical value or less.