The first MODIS instrument was launched aboard NASA's EOS Terra spacecraft in December 1999. MODIS has 20 reflective solar bands (RSB) with wavelengths from 0.41 to 2.2 mum and nadir spatial resolutions of 0.25 km, 0.5 km, and 1.0 km. It also has 16 thermal emissive bands (TEB) with wavelengths from 3.7 to 14.4 mum, all with 1.0 km spatial resolution. MODIS RSB on- orbit calibration and characterization are performed periodically using a solar diffuser (SD) and a solar diffuser stability monitor (SDSM). Another on-board calibrator, a spectro-radiometric calibration assembly (SRCA), is used for the sensor's spatial and spectral characterization. In addition to the SD/SDSM system, regularly scheduled lunar observations via spacecraft maneuvers are added to monitor RSB radiometric calibration stability. This paper provides an overview of Terra MODIS RSB calibration methodologies, operational activities, and on-orbit performance from its observations over 6 years. On-orbit results show that the Terra MODIS RSB calibrators have been performing well in tracking changes of detectors' responses and producing useful data sets to maintain RSB calibration and associated data product quality.