Based on the chlorophyll-a (Chl-a) concentration from Sea-Viewing Wide Field-of-View Sensor (SeaWiFS), the intra-seasonal variability of the high Chl-a concentration strip in the western South China Sea (SCS) during summer of 2000 is investigated. In summer, controlled by the eastward current between the eddy pair off the Vietnam coast, the phytoplankton and the nutrients are horizontally advected offshore into the open SCS, which contributes to forming the high Chl-a concentration strip. The high Chl-a concentration strip has a good agreement with the direction of the eastward current. Our analysis indicates that in the western SCS, the summer monsoon occurs over the south of 12°N leading by approximately one week plays a dominant role in the distribution of the high Chl-a concentration strip. Further analysis reveals that influenced by Madden-Julian Oscillation (MJO), the development of monsoon is not a smooth seasonal process but consists of an intense intra-seasonal event. During a typical intra-seasonal event, the enhanced southwesterly monsoon appears to cause the northeastward current to flow eastward, and the high Chl-a concentration strip extends eastward offshore from the coast; As the zonal wind decreases, the offshore current turns northeastward. Consequently, the high Chl-a concentration strip shows northeastward offshore distribution.