The Moderate Resolution Imaging Spectroradiometer (MODIS) is one of the key instruments for NASA's Earth Observing System (EOS) missions. Its high quality land, ocean, and atmosphere data products heavily rely on accurate on-orbit calibration and characterization. MODIS reflective solar bands (RSB) on-orbit calibration is reflectance based and is provided by an on-board Solar Diffuser (SD). The Bi-directional Reflectance Factor (BRF) of the SD was characterized pre-launch. The earthshine (ES) contamination to the SD signal was estimated to be approximately 0.5%. On-orbit observations and geometric optical modeling results show that this contamination strongly depends on the wavelength, calibration viewing geometry, and the Earth surface type. In this paper, the contribution from the earthshine on MODIS RSB calibration is illustrated and characterized systematically. A simple approach is proposed to reduce its impact on the RSB calibration. This approach has been applied to both Terra and Aqua MODIS RSB calibration and proved to be effective in reducing the earthshine contaminations. Consequently, it is recommended for future Level 1B calibration and production improvements.