In this study exposure assessment of welding fume for the workers in welding processes of a transformer manufacturing factory was conducted. A total of 10 workers was recruited and classified into 5 similar exposure groups (SEGs) according to the work areas and tasks the workers performed. The exposure measurements of welding fume included: (1) to use an IOM and nylon cyclone for inhalable and respirable dust sampling respectively; (2) to use a Marple personal cascade impactor for measuring particle size distribution; and (3) to use a scanning mobility particle sizer (SMPS) for measuring environmental particle size distribution. All collected samples were analyzed for iron, chromium, nickel and lead by atomic absorption spectrometry (Varian AAS 220FS, Astralia) with a flame or graphite tube atomizer. The characteristics of welding processes, worker operation and workplace were observed and recorded for the difference exploration of exposure assessment during the period of exposure sampling. The results of exposure data analyses showed that the proportions of iron, chromium, nickel and lead in the respirable dust to those in the inhalable dust were 50%, 88%, 88% and 77%, respectively. This indicated the welding fume exposure of the workers could not be ignored. From the exposure comparison of the SEGs, individual workers could have high exposure to a specific metal due to the performance of some specific tasks, for example, the workers in the SEG involving the welding of nickel content parts received nickel exposure exceeding the NIOSH recommended expoure limit 15 µg/m3 as well as the workers in the SEG involving the welding of lead pipe were exposed to lead greater than the occupational exposure limit 50 µg/m3 suggected by ACGIH. Because significant differences of metal exposure were observed in the workers of the same SEG, it was difficult to maintain the exposure similarity of the SEGs constructed for the exposure sampling. More studies are necessary for the construction of stable SEGs for the purpose of exposure assessment. Based on the results of personal measurements of particle size distribution, it demonstarted that the majority of the workers were exposed to the particles in diameter less than 1 µm and the mass proportion of the respirable particles was 30~50% of the whole collected particles. The particle size scanning obtained from the SMPS also showed a large amount of particles in diameter less than 1 µm. The exposure control measures of weldin fume for the workers should concern the removal of respirable and even finer particles.