A key instrument for the NASA Earth Observing System (EOS) mission, the Moderate Resolution Imaging Spectroradiometer (MODIS), is currently operating on-board the Terra and Aqua spacecrafts. This paper discusses the calibration uncertainty analysis for the MODIS Reflective Solar Bands. Each MODIS, either on the Terra or on the Aqua spacecraft, has 20 reflective solar bands, making observations at three different nadir spatial resolutions: 250m (B1-2), 500m (B3-7), and 1000m (B8-19, and B26). The 250m, 500m, and 100m bands have 40, 20, and 10 detectors per band, respectively. The reflective solar bands spectral wavelengths are between 0.41 and 2.3 μm. On-orbit, a solar diffuser is used for the reflective solar bands calibration. For the high gain ocean color bands (B8-16), a retractable attenuation pinhole screen is placed in front of the solar diffuser during each calibration. For the reflective solar bands, the specified uncertainty at the typical scene is 2% in reflectance and 5% in radiance. The uncertainty analysis to be presented in this paper will include the approaches and estimated results for Terra MODIS. Aqua MODIS L1B uncertainty is not reported but is extremely similar to Terra. Emphasis will be on the solar diffuser bi-directional reflectance factor characterization at pre-launch since it is a major contributor to the reflective solar bands uncertainty. Other factors include the Earth view response-versus-scan angle, solar diffuser degradation and attenuation screen effect. For the Terra MODIS instrument, the estimated uncertainties based on the instrument characterization and performance will be compared with the specifications.