Aerosol, which is tiny particle in the atmosphere, affects the microphysical and radiative properties of cloud particles. This effect is called “cloud-aerosol interaction”, which is one of the most uncertain factors for climate predictions. Cloud microphysical model and aerosol transport model, which are parts of component in atmospheric numerical simulation code, have been developed and used to understand the cloud-aerosol interaction. In my talk, I first introduce the cloud-aerosol interaction, the cloud microphysical model, and the aerosol transport model. After that I will introduce several examples about how to use the cloud microphysical model and aerosol transport model for understanding the cloud-aerosol interaction.