This paper deals with an investigation of the road roughness on the vehicle ride comfort using semi-active suspension system. A mathematical model of quarter vehicle for semi active suspension system is developed to evaluate vehicle ride comfort. The rolling resistance and power losses are also investigated. The power consumed in rolling resistance and power dissipation in suspension for passive and semi-active suspensions are evaluated. The obtained results showed that ride comfort increases as the road roughness is decreased. Comparisons between passive and semi-active suspensions systems in terms of ride performance and power dissipation are also discussed.