The objective of this study is to develop a non-steady & non-linear tire model for vehicle dynamic simulation and control for extreme lateral slip condition. This model is provided in a semi-analytical form based on the theoretical non-steady state model, presented at 2nd IAVSD Tyre Conference, Feb. 1997. The tire model is based on a quasi-steady state concept, which generates the dynamic forces and moment according to the dynamic effective slip ratio cooperating with the Unified Semi-Empirical Tire Model for Steady State. Satisfying the theoretical boundary conditions at two sides (lowest & highest) in frequency domain, the tire model is capable of describing the transient force & moment characteristics of tires in higher frequency range, Comparing with the “Linear Approximation” model, presented at 4th AVEC Conference, Sept. 1998. Validated against a large amount of measurement data, It is confirmed that the model is able to reflect the transient tire characteristics with sufficient accuracy for higher frequency and larger input ranges.