In this work, multiple transitions from isothermally crystallized poly(3-hexylthiophene) (P3HT) and their correlations to the structure were investigated. It was discovered that high and low temperature crystallites of varying degree orders can be formed in P3HT matrices during the isothermal crystallization. The high temperature crystallite was highly ordered crystalline form having melting temperature around 230 degrees C; while the low temperature crystallite was quasicrystalline of limited ordered having a varying endothermic transition temperature depending on crystallization conditions. The population of high temperature crystallites can be manipulated by the variation of isothermal crystallization conditions. The small angle X-ray scattering revealed a progressive phase separation at appropriate crystallization temperature and/or prolonged crystallization times. (C) 2016 Elsevier Ltd. All rights reserved.