This paper describes automobile collision avoidance system by using of an ultrasonic sensor for a vehicle. We utilize the electronic systems application embedded in car that is anticipated to minimize the disaster of car accident. This paper is concentrating on developing a model of rear end car collision avoidance system that detects the gap among motors moving in the identical lane, inside the identical direction and alert the driver each time she or he is in danger range by using a microcontroller. The gap is measured via an ultrasonic sensor used to experience the obstacle beforehand.