As a substitution material for gold, aluminum and especially its nanocluster form deposited on a clean silicon surface are important for the electronic industry. In this article, a scanning tunneling microscope has Been used to deposit nanoscale aluminum clusters on a clean stepped Si(lll) surface by applying bias pulses to the tip-sample tunneling junction. It has been found that thresholds for the bias pulses exist for the deposition of aluminum nanoclusters for both positive and negative voltages. By operating around these thresholds, rather small (down to 3 nm) aluminum clusters can be generated. However, positive and negative bias pulses tend to give different nanocluster types, and this can be explained in terms of field evaporation mechanism by considering the evaporation field intensity changes during the deposition process.