A fundamentally based quasi-dimensional NOx emission model for spark ignition direct injection (SIDI) gasoline engines was developed. The NOx model consists of a chemical mechanism and three sub-models. The classical extended Zeldovich mechanism and N₂O pathway for NOx formation mechanism were employed as the chemical mechanism in the model. A characteristic time model for the radical species H, O and OH was incorporated to account for non-equilibrium of radical species during combustion. A model of homogeneity which correlates fundamental dimensionless numbers and mixing time was developed to model the air-fuel mixing and inhomogeneity of the charge. Since temperature has a dominant effect on NOx emission, a flame temperature correlation was developed to model the flame temperature during the combustion for NOx calculation.Measured NOx emission data from a single-cylinder SIDI research engine at different operating conditions was used to validate the NOx model. The effects of fuel injection timing, injection pressure, spark timing, overall engine AFR, and intake temperature on NOx emission were examined and well captured by the model. Comparison of all the NOx emission data with the model indicates that the NOx model is a good predictive tool for NOx emissions in SIDI engines.