Nucleation and growth of three-dimensional (3D) micrograins formed during constant surface pressure relaxation of supersaturated arachidic acid monolayers are studied by atomic force microscopy (AFM). The morphologies of the surface layers at different stages of the 2D-3D transformation kinetics are visualized after transfer onto glass substrates. Growth and distribution of the 3D micrograins are quantitatively determined by a grain size analysis, providing number, average cross sectional area, and height of the grains. The application of the nucleation-growth-collision theory to the experimental data obtained by the grain size analysis of AFM and the Constant surface pressure relaxation verifies the limiting case of progressive nucleation.