Crystallization behavior of coal slag influences the slag flow and rheology properties in the slagging gasifier. Crystallization behavior of coal slag during isothermal molten process was observed and analyzed on the in situ experiment study. Complex aluminosilicates reaction led crystal to separate out in molten slag, mainly including the hexahedral crystal and acicular crystal, which varied with slag molten temperatures. The growth of crystal size first increased and then remained stable on molten slag surface with increasing isothermal time. The distribution and migration of coal slag elements on slag surface and inside crystal were also detected by time of flight secondary ion mass spectrometry (ToF-SIMS). Al, Ca and Si homogeneously distributed on the crystal and amorphous phase, due to the high content of element in coal slag. K mainly enriched on the crystal surface and amorphous phase while Na diffused and migrated onto the crystal surface. Mg doped into the interior of crystal, and the distribution of Fe varied with crystal type. (C) 2016 Elsevier Ltd. All rights reserved.