In situ buoy observation data spanning four years （2008-2011） were used to demonstrate the year-to-year variations of the monsoon onset processes in the Bay of Bengal （BOB）. A significant early （late） monsoon onset event in 2009 （2010） was analyzed in detail. It is found that the year-to-year variations of monsoon onset can be attributed to either the interannual variability in the BoB SST or the irregular activities of the intra-seasonal oscillation （ISO）. This finding raises concern over the potential difficulties in simulating or predicting the monsoon onset in the BoB region. This uncertainty largely comes from the unsatisfactory model behavior at the intra-seasonal time scale.