Abstract: In this paper, an investigation on the factors affecting the formation of acicular ferrite in C-Mn-Si weld metals under the shield of CaF₂ with simulated weld metal melted by TIG arc was described. The results indicated that a high volumetric ratio of acicular ferrite in the simulated weld metal can be obtained in case the ratio of manganese and silicon is in a certain range. The nucleating centers are the non-metallic inclusions in the larger bainite grains. A high volume fraction of dispersive inclusions of small size (generally less than 1μm) in the weld metal is good for the formation of acicular ferrite. Micro-quantity of Ti in the weld metal may increase the probability of acicular ferrite formation.