Identical twin experiments with and without topography are conducted with the Weather Research and Forecasting (WRF) model in an idealized framework to investigate the impact of topography on the initial error growth associated with moist convection. A topography is set as a single Gaussian shape mountain with a peak height of about 1000 m. Both experiments show clear diurnal cycles, while moist convection develops earlier and organizes to a larger size in the experiment with topography. To evaluate the initial error growth, a metric referred to as moist difference total energy (MDTE) is proposed to represent the differences between the two simulations in twin experiments. The horizontal spatial distribution, temporal evolution, and horizontal wavenumber space of the MDTE suggest that the error growth is greatly dominated by the different features of convection development between the two experiments. The analysis based on individual cloud areas shows that the convective clouds developing over the mountain have smaller MDTE at the early stage of development.