The effect of secondary air injection (SAI) on exhaust hydrocarbon (HC) emission has been investigated in a spark-ignition (SI) single cylinder engine operating at steady-state cold condition. Both continuous SAI and synchronized SAI, which corresponds to intermittent secondary air injection to exhaust port, are tested. Oxidation characteristics of HC are monitored with a FID analyzer and exhaust gas temperatures with thermocouples. Effects of exhaust air-fuel ratio (A/F), location of SAI, and engine-A/F have been investigated.Results show that HC reduction rate increases as the location of SAI is closer to the exhaust valve for both synchronized and continuous SAIs. HC emission decreases with increasing exhaust-A/F when exhaust-A/F is rich, and is relatively insensitive when exhaust-A/F is lean. In synchronized SAI, SAI timing has significant effect on HC reduction and exhaust gas temperature. Optimum SAI timing observed is ATDC 100° and 230°. Synchronized SAI is found to be more effective in HC reduction compared to continuous SAI. HC emission for a typical cold start condition decreases about 77% and 23% with synchronized and continuous SAIs, respectively, compared to that with base engine condition, which has no SAI. In addition, the exhaust gas temperature with synchronized SAI is much higher than that with continuous SAI and base engine condition, especially near the exhaust port exit.