This paper proposes the estimation algorithm of road conditions from tire vibration. The basic concept of estimation is that the tire vibration is determined by road irregularity and road friction condition. Because of difficulty of direct measurement of tire vibration, it is estimated from body acceleration using known tire model and vehicle properties between tire and body. To estimate the road irregularity, the Independent Component Analysis (ICA) method is used. The elastic deformation of the tire belt is modeled using the finite element absolute nodal coordinate formulation which allows for modeling large rotational motion and the nonlinear inertia effects. With comparison between estimated tire vibration from road irregularity with ICA and tire model and actual tire vibration from road irregularity and road surface condition with known vehicle properties, road surface condition could be estimated. So, tire vibration power spectral density (PSD) calculated by using the transmission characteristics becomes the function of the road surface, the road ruggedness, and the tire characteristic. These are clarified by the PSD ratio, ICA and the tire model, and road surface condition become able to estimate.