In this paper, Smart Parking Assist System(SPAS) using ultra-sonic sensor is described. The algorithm of SPAS and advantages of newly developed algorithm are introduced. The SPAS algorithm consists of 3parts which are parking space recognition, vehicle position observer and trajectory control algorithm. This algorithm have been implemented and proved in parallel parking scenario using dSPACE Autobox in vehicle. The results show faster parking and reliable performances compared to SPAS of competitors.