Abstract-This work examines the effect of synchronisation error on the performance of spatial modulation (SM) technique in optical wireless communication (OWC) systems. SM exploits the deployment of multiple transmitters by encoding user information on their spatial domain. In most works related to SM, a perfect synchronisation among these multiple transmitters is assumed. However, synchronisation error can result from multipath propagation in OWC channel, and clock jitter and variation in propagation delay of each transmitter. Such error in synchronisation degrades system performance and hence the need to investigate its effect. Using union bound technique, and defining synchronisation errors as timing offsets in the received signals, we derive the symbol error rate for space shift keying (SSK), generalised SSK (GSSK), SM and generalised SM (GSM) schemes, and we validate our analysis with tightly-matched simulation results. Results show degradation in performance increases with synchronisation error. While SSK is tolerant for a small range of synchronisation error, GSSK, SM and GSM are significantly impaired. Our results also demonstrate the dependence of SM on channel gain values. We observe that the lower the channel gain of the transmitter in which synchronisation error occurs, the lesser the impact of the synchronisation error on the system performance.