Different crystalline forms are obtained by simply manipulating the good/poor solvent ratio in mixed solvents of toluene/ethanol. Depending on different solvent ratios, poly(9,9-dioctylfluorene)s (PFOs) can generate a large palette of morphologies including lenticular crystals, fibrous crystals and rod-like crystals. In the crystallization process, polymer chains experience different kinetic pathways, yielding lenticular crystals in the toluene solution, rod-like crystals at a low toluene/ethanol ratio (1:0, and fibrous crystals at a high toluene/ethanol ratio (3:1). A combination of atomic force microscopy (AFM) and transmission electron microscopy (TEM) provides an opportunity to elucidate the distinctive molecular arrangements in these different crystals. Moreover, the influence of molecular weights on the crystallization of polymers has also been investigated in different crystals.