Abstract: The physical properties of organic films depend crucially on their orientations and long-range ordering. A thorough understanding of the growth mechanism of organic semconductors is of great importance. By studying the growth processes of two typical systems, perylene on Ru(0001) and tetracene on Ag(110), the growth behavior of organic semiconductors and the possibility for formation of the organic crystalline films are addressed. For perylene on Ru(0001), the results show a formation of the Ru(0001)- 12×12-8 perylene superstructure, and the growth behavior is mainly controlled by the laterally repulsive molecule-molecule interaction. However, in the case of tetracene on Ag(110), the main driving force is the laterally attractive molecule-molecule interaction, so the tetracene grows in a 2D island mode in the monolayer regime but for thicker multilayers a crystalline film in a novel 3D orthorhombic structure is formed layer by layer.