Abstract: Objective To evaluate the cone beam CT (CBCT) image quality and its long-term stability for HUs under different scanning conditions. Methods With the OBI system, Catphan 504 phantom using the different modes was scanned through one year. Analysis of variance (ANOVA) was used to evaluate the CBCT images. Results The results of contrast resolution under different scanning conditions met the requirements of acceptance test procedure. Spatial contrast resolution(≥ 5 lp/cm) of half-fan scanning conditions were inferior to those (≥ 7 lp/cm) of full-fan mode. The accuracies of spatial linear distance and angle were within ±1%. HUs in air density plug were constant, but HUs of other plugs with different conditions had the significant difference (F=660.84, P=0). Under different conditions in a year, the mean HUs showed a good agreement. Conclusions HUs of CBCT image under different scanning conditions exist a little difference, but there is a good consistency for the long term stability analysis. It is helpful for CBCT images directly used for dose re-calculation in adaptive radiation therapy (ART).