Spatial Modulation(SM) is a newlyintroduced wireless transmission scheme,andSpace Shift Keying(SSK) is its simplifiedversion.Their capacity performance has notbeen well examined to date.Starting frombasic definitions of information theory,through theoretical analyses and numericalcalculations,this paper evaluates the capacityof SM systems in various channels.Our conclusion is that under Rayleigh fading channels,the capacity of the SSK system is limited bythe Signal-to-Noise Ratio(SNR) or the number of transmit antennas,whichever one issmaller.From the capacity point of view,theSM system is better than the Single-Input andSingle-Output(SISO) system,but worse thanthe Multiple-Input and Single-Output(MISO)system.The correlation between transmit antennas will degrade the performance of theSSK system.Line-of-Sight(LOS) may causeperformance degradation for SSK,but capacity increases for SM. Spatial Modulation (SM) is a newly introduced wireless transmission scheme, andSpace Shift Keying (SSK) is its simplified version.Their capacity performance has not been well examined to date. Starting frombasic definitions of information theory, through theoretical calculations and numericalcalculations, this paper evaluates the capacityof Our systems are various channels. Our conclusion is that under Rayleigh fading channels, the capacity of the SSK system is limited by the Signal-to-Noise Ratio (SNR) or the number of transmit antennas, whichever one issmaller. From the capacity point of view theSM system is better than the Single-Input and Single-Output (SISO) system, but worse thanthe Multiple-Input and Single-Output (MISO) system. The correlation between transmit antennas will degrade the performance of theSSK system. Line-of- Sight (LOS) may cause performance degradation for SSK, but capacity increases for SM.