Steady-state visual evoked potentials (SSVEP) are increasingly used in the development of brain-computer interface techniques (BCI). We investigated the spectrum differences of three kinds of flickers and the differences in SSVEPs evoked by three different stimulators, i.e. the light-emitting diode, the cathode ray tube of a desktop monitor and the liquid crystal display of a laptop screen. The results showed that the SSVEP differences were strongly related to the frequency spectrum differences of the flickers. According to these differences, the stimulator was selected based on the complexity of the BCI system.