The growth behavior of metallic aerosols under normal and reduced pressure environments was investigated. Particles grown at both environments exhibit very irregular profiles. Particles' growth, in general, can be categorized into three types in this study, they are: cluster-cluster aggregation, particle-cluster aggregation, and ballistic aggregation. A descriptor, called fractal dimension, is applied to quantifying the effect of reducing the pressure on the growth of particles. According to the measurements of fractal dimension, at higher pressure, 0.1 and 1.0 atm., cluster-cluster aggregation occurred at the early stage of growth; while particle-cluster aggregation occurred at the later stage of growth. Opposite phenomenon was found when pressure decreased to 0.01 atm.