Humans have been influencing climate by changing the atmospheric composition of greenhouse gases and aerosols in the atmosphere. The effects of greenhouse gases have been well understood in global scale. However, the magnitude of its effects on regional scale is still uncertain because of the contaminations from other human activities such as changing land use and generating aerosols. Few studies have been focused on California. It has been shown that the temperature has been experiencing a cooling trend in the last decades in California. Here, we employ a regional climate model with aerosol data from a chemistry transport model to investigate the effects of aerosols (including sulfate, soot, organic carbon, sea salt and dust) on the regional climate in California. In our study, the primary concerns will the effects of aerosol on direct radiative forcing and the surface temperature change during summer season. We believe that our work will help the understanding of the magnitude of aerosol effects on regional climate of California, and the relative contributions of different aerosol species. Our efforts can also help us re-estimate the magnitude of the greenhouse gases effects with more confidence in this region.