Nanoscale chemical identification required for the analysis of functional materials with fine structures. For the first time, high-resolution tip-enhanced Raman mapping (TERM) was applied to a polymer system poly(methyl methacrylate) (PMMA)/poly(styrene-co-acrylonitrile) (SAN) thin films. Before TERM measurements were performed, the linear enhancement of tip-enhanced Raman spectroscopy (TERS) was optimized in of maximum Raman intensity. Up to 15 times of linear enhancement was obtained, as compared to the (conventional) confocal Raman intensity:The,enhancement factor of TERS is greater than 1500 if taking the 100 times smaller probing area into account. As a result, a short exposure time was sufficient for high-resolution TERM measurements. Using TERM, the phase separation behavior of PMMA/SAN thin films was monitored by chemical recognition of local composition. The interface width (similar to 200 nm) at the early stage of phase,evolution was visualized. The comparison of TERM images at different stages of the phase separation process revealed an unexpected transition of PMMA from the dispersed phase to the continuous phase:This morphology transition of PMMA/SAN is briefly discussed in terms of the glass transition temperature, interface, and surface tension.