The Madden Julian Oscillation (MJO) is the most prominent mode of intraseasonal variations in the tropical region. It plays an important role in climate variability and has a significant influence on medium-to-extended range weather forecasting in the tropics. Since its discovery by Madden and Julian over three decades ago, MJO has continued to be a topic of significant interest due to its complex nature and the wide range of phenomena it interacts with. In contrast to observational studies, progress in numerical simulations of MJOs is much slower, particularly in global climate models (GCMs). GCMs have had difficulty in simulating the observed characteristics of the MJO. Recently, we revised the Zhang-McFarlane convection scheme in the NCAR CCM3 to address the model’s deficiency in simulating the intraseasonal variability and MJO. In comparison with the observation and the standard CCM3 simulation, we find that the revised Zhang-McFarlane scheme produces a much-improved simulation of the intraseasonal variability and MJO. For instance, the surface wind and precipitation patterns in the composite MJOs simulated by the revised Zhang-McFarlane scheme are in better agreement with the observations than the standard CCM3 in many important aspects, including the amplitude and eastward propagation characteristics. Examination of convective heating from both deep and shallow convection indicates that near the mature phase of the MJO, shallow convection develops ahead of the deep convection. This is consistent with the recent observations and theoretical development that shallow convection helps to precondition the atmosphere for MJO by moistening the lower troposphere.We are currently investigating the possible mechanisms of the MJO in the model to understand what is responsible for the improved simulation of MJO. Details will be presented at the meeting.