Blending conjugated polymers is an efficient method to improve the properties of the films. The phase diagram of poly(9,9-dihexylfluorene) (PF) and poly(2-methoxy-5-(2'-ethyl-hexyloxyl)-p-phenylene vinylene) (MEHPPV) was predicted by a modified Flory-Huggins theory based on the topological method (graph theory) for the structure-property correlations. It shows that the two polymers have a strong trend to separate. Atomic/friction force microscopy (AFM/FFM) measurements show there exist microphase separations in film prepared at room temperature. After annealing at 160 ° C, serious phase segregations took place in both the lateral and vertical direction. The photoluminescence of the thin films was also measured by a fluorophotometer. © 2005 Wiley Periodicals, Inc.