Several novel phenomena of phase-separated Langmuir-Blodgett films are revealed by atomic force miscroscopy : (a) the existence of grains with a diameter of 20-100 nm in the liquid expanded phase; (b) the existence of a rough boundary for the liquid condensed domains; (c) an inhomogenous distribution of hole-like defects in the liquid condensed domains. The results are relevant to understanding the early stages of the domain formation and the defect formation in Langmuir monolayers.