In a series of studies on retardation of water evaporation by the binary mixed monolayer of poly (vinyl long-chain ester) and octadecanol (OA), microstructures obtained by a Brewster angle microscopy (BAM) and atomic force microscopy (AFM) images have been investigated with varying the composition of mixed monolayer of poly (octadecylacrylate) (POA) and OA. The mixed monolayer of POA/OA = 1:3-1:4 exhibits considerably effective resistance to water evaporation at 20-40 mN/m, which is reflected in the condensation effect by mixing on the pi-A isotherms. From the BAM image of each monolayer at zero pressure the rather heterogeneous film was obtained for the single component and the homogeneity of the monolayer was apparently improved by their mixing. The flattest BAM image could be obtained for the mixed monolayer of POA/OA = 1:3 in the submillimeter scale. This result has good correspondance to the retardation effect of the mixed monolayer on water evaporation. On the other hand, for the AFM image of POA film some defects with depth of about 1.9 nm were observed and small ones still remained by adding OA in the nanometer scale.