Intragranular acicular ferrite is regarded as a most desirable microstructure feature, in view of its strength and toughness, both in weld metals and in the heat-affected zone. This paper systematically investigated the effect of Ti addition on the evolution of intragranular acicular ferrite in the heat-affected zone of C-Mn steel. We also systematically studied the effects of austenite grain size, alloy content and the characteristic of inclusions on the formation of intragranular acicular ferrite. The nucleation and growth of intragranular acicular ferrite was directly observed by laser scanning confocal microscopy. Subsequently, microscopy analysis was used to quantitatively determine and distinguish the potent and inactive inclusions with respect to the nucleation of intragranular acicular ferrite. Finally, some possible reasons are given to explain the formation of intragranular acicular ferrite in the C-Mn steel. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.