Abstract: It was demonstrated by electron diffraction patterns that lead sulfide (PbS) semiconductor particles films were obtained under arachidic acid monolayer, floating on lead nitrate solution, by injection of S2－ aqueous beneath the monolayer. Different shapes of the PbS semiconductor nanocrystallites, such as triangle, quadrangle and wooden club, were formed under different surface pressure of monolayer. The influence of surface pressure of monolayer preparation of arachdic acid on the structure of PbS nanocrystallites was studied by TEM images and electron diffraction patterns. Controlling surface pressure at 25 mN•m－1, the epitaxial growth of PbS nanocrystallites with their (111) planes parallelling to the (110) planes of monolayer, was proved by TEM images and electron diffraction patterns. A mechanism for the PbS nanocrystallites epitaxial growth was rationalized in terms of lattic matching between the arachidic acid headgroups in the monolayer and the semiconductor nanocrystallites, which were fabricated under monolayer.