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The nucleation of ordered phase was simulated based on microscopic diffusion equation and the assumptions of the classical nucleation theory were examined. The quantitative calculations of interface thickness evolution were accomplished for the first time. It was found that the interfaces between ordered phase and disordered matrix were diffuse. The interface thickness decreased with time, from the initial 1.2 nm to an equilibrium value 0.6 nm. The ratios of the radius of ordered particles and the interface thickness monotonously increased, but they were of the same order of magnitude all the time. The sharp interface assumption should not be adopted in this stage. For the Al-10%Li (atom fraction) alloy aged at 192℃, the assumptions of the classical nucleation theory disagreed with the facts. The phase transformation followed the non-classical nucleation mechanism and the applicable scope of the classical nucleation should be confined.