The fast luminosity monitor counting the γ photons above a given energy threshold emitted from radiative Bhabha scattering has been operated in the BEPCII to measure the relative luminosity bunch by bunch for the first time and used successfully in beam tuning of BEPCII. In the relative mode the monitor is able to deliver the relative luminosities with an accuracy of 0.8%. By steering the electron beam while observing the counting rate changes of the monitor the horizontal and vertical sizes of the bunch spots can be estimated as: sxe+ = sxe– = 0.356 mm, sye+ = sye– = 0.011 mm.

The BEPCII Luminosity Monitor (BLM) monitors relative luminosity per bunch. The counting rates of gamma photons, which are proportional to the luminosities from the BLM at the center of mass system energy of the ψ (3770) resonance, are obtained with a statistical error of 0.01% and a systematic error of 4.1%. Absolute luminosities are also determined by the BESIII End-cap Electro-Magnetic Calorimeter (EEMC) using Bhabha events with a statistical error of 2.3% and a systematic error of 3.5%. The calibration constant between the luminosities obtained with the EEMC and the counting rates of the BLM are found to be 0.84±0.03 (×1026 cm−2·count−1). With the calibration constant, the counting rates of the BLM can be scaled up to absolute luminosities.