Recent observations have revealed that there are intraseasonal oscillations (ISO) of convection over South America (SA). A reconstruction of the flow pattern in association with the SA ISO suggests that their sources could be (1) the Madden-Julian Oscillation (MJO) from the Western Pacific and (2) a local forcing mechanism (Zhou and Lau, 1999). In the present study, the physical mechanism of the SA ISO has been investigated by using the National Center for Atmospheric Research (NCAR) regional climate model (RegCM2). The National Centers for Environmental Prediction (NCEP) reanalysis was used to provide initial conditions and time-dependent or time-averaged lateral boundary conditions for the model integration. Our results indicate that the SA ISO still exists with time-averaged lateral boundary conditions, which prevent the MJO and other outside disturbances from entering the model's domain, suggesting a local forcing mechanism for the SA ISO in addition to the MJO. Further experiments show that the SA ISO still exist even with temporally constant radiative cooling rates, a result distinct from the theory proposed by Hu and Randall (1994).