The characteristics of the 30—60 d intra-seasonal oscillation(ISO) over the South China Sea(SCS) and its relationship with ENSO cycle are diagnosed by using the NCEP/NCAR reanalysis data,NOAA outgoing long wave radiation(OLR) data and extended reconstructed sea surface temperature from 1979 to 2009.The results are as follows:(1)The 30—60 d period of the ISO over the SCS is significant from May to October.The ISO over the SCS during normal years,which is represented by the convection in company with the ISO,presents eastward and westward longitudinal propagation and seasonal latitudinal sway between the two hemispheres.The ISO intensity over the SCS represented by the low frequency kinetic energy(KE) shows interdecadal variation with an increasing trend during the past 31 years as well as monthly change with a peak from July to August.(2)The ENSO cycle has a significant influence on the ISO convection over the SCS,and the convection intensity tends to be suppressed(intensified) during El Niño(La Niña) years.In contrast with normal years,the westward propagation of the ISO convection over the SCS weakens while the northward spread is notable during both El Niño and La Niña years.The northward spread is characterized by the northward leap of active convection band from the Southern Hemisphere to the Northern Hemisphere in the spring(from April to May),and the band can reach 35°N during La Niña years.However,the spread direction in the Northern Hemisphere is distinctly different from that in the normal years around September.(3)The ISO intensity over the SCS has a close relationship with ENSO cycle.The ISO intensity would weaken(strengthen) during El Niño(La Niña) years and has about half a year(6—8 months) lag correlation with the occurrence of mature El Niño(La Niña) event.The correlation between ISO intensity and the sea surface temperature anomaly over the Niño3 region is the most striking over the western and central Pacific in the spring and autumn before the mature El Niño occurs.The correlation centers also spread eastward accompany with the high value centers of low frequency KE.