Neck Injuries, referred to in this paper as whiplash are among the most frequent injury among occupants in low speed rear end car collision. This paper analyzes the correlations between influence parameters of head restraints and whiplash injury criteria. In this paper, we used DFSS (Design for Six Sigma) method for design optimization of head restrains. Four control factors of head restraints have been selected by function matrix method. The effects of the control factors have been experimentally evaluated by using a sled pulse from 16kph relative velocity which is suggested by Korean New Car Assessment Program (KNCAP). Whiplash tests were repeated once in order to reduce the noise factors of dynamic assessments. By using DFSS, the correlation between control factors of head restraints and injury criteria has been comprehended.