A statistical analysis on the radiation belt dropouts is performed based on 4 years of electron phase space density data from the Van Allen Probes. The μ, K, and L* dependence of dropouts and their driving mechanisms and geomagnetic and solar wind conditions are investigated using electron phase space density data sets for the first time. Our results suggest that electronmagnetic ion cyclotron (EMIC) wave scattering is the dominant dropout mechanism at low L* region, which requires the most active geomagnetic and solar wind conditions. In contrast, dropouts at high L* have a higher occurrence and are due to a combination of EMIC wave scattering and outward radial diffusion associated with magnetopause shadowing. In addition, outward radial diffusion at high L* is found to cause larger dropouts than EMIC wave scattering and is accompanied with active geomagnetic and solar wind drivers. Plain Language Summary Radiation belt dropout is one of the most dramatic variations in Earth's magnetosphere, which means the relativistic electron fluxes can decrease a few orders in just several hours. Two mechanisms have been proposed to explain the quick depletion of radiation belt electrons: loss through magnetopause and the precipitate into the atmosphere. However, their relative contribution is still not clear now. In this paper, we use the 4‐year measurement of Van Allen Probes to investigate the statistical features and underlying physical mechanisms of radiation belt dropouts. Through using electron phase space density rather than electron flux, we reveal the real loss of electrons in the radiation belt. The results show that electrons at low L* are mainly precipitated to atmosphere while high L* electrons both loss to magnetopause and atmosphere. Key Points EMIC wave scattering is the dominant dropout mechanism at low L*, accompanied with the most active geomagnetic and solar wind conditions Dropouts at high L* have a higher occurrence and are due to a combination of EMIC wave scattering and outward radial diffusion At high L*, outward radial diffusion causes larger dropouts than EMIC wave scattering, and it favors more active conditions