Spectroscopic Ellipsometry (SE) was chosen to study thin film growth in atomic layer deposition (ALD). It was shown that Cauchy model had limitations in predicting the ultrathin film thickness at initial few deposition cycles, and the fitting results depend on wavelengths range greatly. Effective Medium Approximation (EMA) model is capable of predicting ultrathin film’s physical properties. Our experiments on Al2O3 growth give supporting evidence on the applicability of EMA model, where it is used to successfully explain the initial nucleation and island like growth. EMA model can be extended to be used for Palladium thin film, which can give reasonable thickness and void content.