Our previous studies of collapsing processes in a stearic acid (SA) monolayer with a Brewster angle microscope have suggested that the collapsing processes can be classified into three basic models : nucleation, straight ridge-like catastrophe, and colloidal particle accumulation. We will provide some more evidence of them with an atomic force microscope. In this paper the nucleation of a SA monolayer on the acidic subphase surface has been studied, and three different nucleation patterns found, which are multi-crystal with a fractal structure, separately distributed cubic crystals, and amorphous nuclei.