Uniform, well-defined star-shaped PbS nanocrystals with tunable sizes (40-100 nm, see Figure), as well as octahedral PbS nanocrystals, have been readily synthesized in aqueous solutions containing a mixture of the surfactants cetyltrimethylammonium bromide/sodium dodecyl sulfate (CTAB/SDS) at low temperature (80 degrees C). The size of the PbS nanostars can be accurately controlled by adjusting the reaction time. This result may open new avenues for the green chemical synthesis of shape-controlled semiconductor nanocrystals.