Abstract: This paper presents an investigation on the welding fume particle size distribution characteristics and its formation mechanism in two different welding techniques: GMAW with H08Mn2Si solid wire and CO₂ shielded FCAW with SJ501 wire. The particle size distributions were measured by quasi-elastic laser scattering and the fume morphology and its chemical composition were examined by TEM and EDS. The experimental results showed that two types of primary particle were spherical shape with smooth surface, whose diameters were generally smaller than 200 nm. The particles were mostly iron oxides. It was also found that secondary fume particles were chain like or mesh like shape, which were agglomerated by a number of primary particles. Compared to the fume collected in GMAW, the chemical composition of FCAW fume was more complex and the secondary particles were smaller.