The NASA/GSFC Data Assimilation Office (DAO) general circulation model (GCM) assimilations (GEOS3) with surface precipitation from satellite passive microwave measurements for May-August 1998 and its control assimilations without the measured surface rainfall will be analyzed in detail to evaluate the impacts of rainfall assimilation on Madden-Julia Oscillation (MJO) and large-scale dynamics. The 40°N-40°S global coverage of rainfall from the Tropical Rainfall Measuring Mission (TRMM) is focused, especially for studying Asia summer monsoon activities. The outgoing longwave radiation (OLR) and ECMWF datasets for the same time period will be also used to investigate MJO characteristics. Results from this study would show positive impacts of measured rainfall on GCM precipitation assimilations and large-scale dynamics. Better MJO features from the GCM data assimilation could lead to future improvements on GCM parameterization schemes and eventually improve GCM performances.