Observational evidence of the life cycle and the northward propagation mechanism of the boreal summertime intraseasonal oscillation is presented using a cyclostationary EOF analysis for the 22-yrs of the NOAA outgoing longwave radiation and the NCEP/NCAR reanalysis data. Time evolution of boreal summer intraseasonal convective anomalies shows the origin of the equatorial Indian Ocean and the characteristic quadrapole-like pattern. It is found that the northward propagation of convection arised from the systematic interaction among convection, surface dynamics, thermodynamics and radiation. Among these, anomalous downward latent heat flux resulting from anomalous surface easterlies north of the equatorial convection in the Indian and western Pacific Oceans is an important factor for the northward propagation. The anomalous latern heat flux is a factor of more than two greater than the anomalous shortwave radiation flux. It is revealed that in the Indian Ocean and subcontinent the second Rossby mode induced by convection plays a major role in the northwestward movement of the convection.