In this report, the morpho-structural peculiarities and the crystallization mechanisms in solutionprocessed, solvent vapor annealed (SVA) thin films of rubrene (5,6,11,12-tetraphenylnaphthacene) on different substrates were investigated. The high-quality rubrene crystal films with a triclinic crystal structure were successfully prepared on the FTO substrates (glass slide coated with fluorine-tin-oxide) modified by PLA (polylactic acid) for the first time. The area coverage of rubrene crystal and the sizes of rubrene dendritic crystals increased with increasing thickness of PLA film and concentration of rubrene solution. For rubrene molecules, FTO wafers with rough surface provided the possibility of heterogeneous nucleation. During the SVA process, there were two kinds of forces acting on the diffusion of rubrene molecules: one force was provided by the residual chloroform solvent, which was perpendicular to the substrate, and the other force was provided by gaseous dichloromethane, which was parallel to the substrate. The synergy of these two forces was proposed to explain the nucleation and the crystallization processes of rubrene films. The higher nucleus of PLA/rubrene dendrites and the layer-by-layer stacking of needle-shaped nanocrystalline PLA/rubrene were important for exploring their kinetic formation process. Organic semiconductor materials, due to their outstanding performance in electronics and optoelectronics, have become the global focus for their promising potential in the research and applications of thin films and flexible electronic device 1-3. Rubrene (5,6,11,12-tetraphenylnaphthacene), as a typical p-type organic semiconductor material, holds a very high record of field-effect mobility (20 cm 2 /V s) 4 in the form of orthorhombic single crystal grown by physical vapor deposition method (PVD) 5-7 , and therefore is used in organic field-effect transistors (OFETs) 8. Besides, rubrene single crystals can also be prepared by "hot wall" deposition method 9 , in situ vacuum annealing 10 , or solution growth methods (such as slow evaporation 11 and cooling from supersaturated solution 12). Rubrene thin films have been deposited by various techniques such as sol-gel coating 13 , combinatorial molecular beam epitaxy (MBE) technology 14 , solution-mediated vacuum deposition 15 , or spincoating 16. In initial works, OFETs based on rubrene thin-film demonstrated a very low mobility (~ 10-6 cm 2 /V s). 17 Then, the mobility of polycrystalline rubrene OFET with hexamethyldisilazane (HMDS) coated SiO 2 dielectric was found to reach up to 10-2 cm 2 /V s 18. For the crystalline rubrene, considered as the active channel in OFETs, a variety of insulating organic polymers and small molecules have been used to induce crystallization of rubrene. Among these, the popular organics were 5, 12-diphenylanthracene and (ultra)high-molecularweight polystyrene (UHMW-PS) 19 , poly(4-vinylpyridine) (P4VP) and poly(methyl methacrylate) (PMMA) 20 , 6,13-pentacenequinone (PQ) 21 , 6,13-diazapentacene (DAP) 22 and so on. In addit...