KMA is operationally monitoring El Niño and La Niña events, which have tremendous impacts on global climate. Many scientific studies have used to define onset of El Niño and La Niña events based on the moving average and persistency of SST indices, and KMA has adopted such definition. Though the definition has been widely accepted, in the operational aspect there is a critical problem to use moving average and condition for the persistence. Because the future values for the SST indices cannot be used in the operational monitoring, the onset timing in El Niño and La Niña can be significantly delayed. We suggest here an appropriate definition of El Niño and La Niña events in the operational aspect. Instead of using the moving average and the condition for the persistence, the onset is defined based on NINO3.4 SST during last 3 months. In order to compare the new definition with the current KMA definition, we applied them to recent 60-years SST data. It is clear that the new definition can declare the onset timing of El Niño and La Niña several months earlier than that of the KMA definition. It suggest that the new definition is more appropriate to the operational monitoring on El Niño and La Niña.