Large-area continuous alpha phase thin films of monodisperse poly(9,9-dioctylfluorene) (PFO), with 32 or 64 fluorene repeating units, named as F32 or F64, respectively, are prepared by melt crystallization. A spherulite-like structure, composing of crystalline nanofibrils, is formed via radial growth after sufficient isothermal treatment. In these nanofibrils, the polymer chains remain extended conformation, which is demonstrated by the similarity between nanofibril widths and extended chain lengths. Comparing this discovery on F32/F64 with our previous observations on F16 (monodisperse PFO consisting of 16 repeating units) leads us conclude that the melt-crystallized phases change from beta' phase (F16) to alpha phase (F32 and F64) as the molecular weight increases. It is speculated that as the number of repeating unit goes up, the backbone planarity of PFO decreases, leading to the formation of different melt-crystallized phases at different molecular weights. (C) 2015 Elsevier Ltd. All rights reserved.