The miscibility behavior and surface morphology of a mixed monolayer, stearic acid (SA) with 5,10,15-triphenyl-20-(4-DL-phenylalanylamindo) phenyl porphyrin (TPPP), were studied by analyzing the surface pressure vs. molecular area isotherms and atomic force microscopy (AFM) images of the transferred monolayer films. The mixed monolayer is a two-phase system with a liquid-condensed phase consisting of SA and a liquid-expanded phase consisting of TPPP with a trace of SA. The different domain structures of SA sensitive to surface pressure, compression rate, annealing, etc. were also studied with AFM in detail.