The authors report on the marked improvement of the quality factor (Q-factor) of single crystal diamond (SCD) nanoelectromechanical system (NEMS) resonators through annealing in oxygen ambient. The SCD NEMS resonators were fabricated by ion implantation assisted technique. The resonance frequency followed well the inverse power law relationship with the length of the cantilevers despite of the annealing. It was observed that there was little modification in the resonance frequency and Q-factor at 430 degrees C, while an obvious red-shift in the resonance frequency occurred at 500 degrees C. Meanwhile, a marked improvement in the Q-factor from around 3500 to 7000 was observed at 500 degrees C. The frequency red shift is due to the etching of diamond with a rate of 0.4-0.5 nm/h at 500 degrees C. The analysis of the energy dissipation discloses that the surface effect dominates the energy loss mechanism for the SCD NEMS resonator. The improvement of the Q-factor is thus attributed to the reduction of the surface defects in SCD NEMS. (C) 2017 Elsevier Ltd. All rights reserved.