Brewster angle microscopic (BAM) study was successfully achieved for a photosensitive monolayer composed of a poly(vinyl alcohol) derivative bearing an azobenzene (Az) side chain. BAM observation revealed clear differences in the morphological and rheological properties between the trans- and cis- isomerized monolayers. The morphological changes accompanied by the UV light induced expansion of this monolayer was successfully observed. An as-spread domain structured inhomogeneous monolayer was converted to a highly homogeneous one after a photochromic cycle.