In this report, a novel in situ method to identify different domains with nanometer resolution is established using AFM equipped with a hot stage. The temporal evolution pattern of surface phase separation and its coalescence process for PMMA/SAN blend thin films have been observed using this technique in real space directly. The kinetics of the surface phase separation studied by this technique, which is very important for us to understand and control the surface morphology and structure, has been discussed. The critical surface phase separation temperature for a 50/50 PMMA/SAN blend film is found to be around 165 °C. From the power law of q*(t)  t-n, two distinct regimes are found through the whole investigated time scale after the thin films abruptly jumped to an unstable state. The early stage probably is dominated by one component’s diffusion to the surface, and the restriction of the surface geometry to chain mobility, resulting in much slower kinetics than those obtained from literature while the late stage obeyed the Lifschitz-Slyozov (LS) law.