Scanning near-field optical microscopy (SNOM) was employed for investigating the phase separation behavior of a polymer blend monolayer which consisted of dye-labeled polp(octadecyl methacrylate) (PODMA) and poly(isobutyl methacrylate) (PiBMA), both forming stable monolayers at the air/water interface. The energy transfer emission was observed in the PODMA rich domain, indicating that there existed the area where both components were mixed at the molecular level. When the polymer blend solution is spread onto a water surface, the PODMA chains tend to aggregate and form a solidlike domain before the PODMA and PiBMA separate completely from each other. This is attributed to the strong crystallinity of the long alkyl group of PODMA. When the temperature was raised to 40 degreesC, the phase separation was almost completed in several minutes, and then the domain size increased with annealing time. The two-dimensional structure for the completely phase-separated monolayer was studied by time-resolved SNOM, A near-field fluorescence decay was observed across the phase interface. The alteration of the fluorescence lifetime at the phase boundary indicated that the interfacial width for the two-dimensionally phase-separated polymer blend was markedly larger than the interfacial thickness expected in three dimensions.