Objective To quantitatively evaluate the image quality, stability and volume precision in kilovoltage cone beam CT (CBCT) on Varian linear accelerator. Methods The Catphan600 phantom was repeatedly scanned in the full-fan and half-fan CBCT scanning modes. A simulation fan-beam CT (FBCT) was used as a benchmark and results related to the low contrast resolution, spatial resolution, uniformity and image noise were compared with the CBCT using the treatment planning system. The comparison of image quality and long-term stability and volume precision was analyzed. Results Spatial resolution was no differences observed between FBCT and CBCT (6 lp/cm∶6 lp/cm,T=18.00, P>0.05). Low contrast resolution was, on average, 1.65% and 1.74% for both CBCT_Full-Fan and CBCT_Half-Fan, and 1.03% for the FBCT (T=6.00, P<0.05). Uniformity was, on average, 0.005 and 0.033 for both, and 0.003 for the FBCT (T=6.00, P<0.05). In terms of image noise, the FBCT images were superior to the CBCT (T=30.00, P<0.05). In valid scan range of the CBCT, reconstructed precision was high. There was no significant time trend in the image quality. Conclusions The image quality of kilovoltage CBCT is inferior to the conventional CT. However, tumor and soft tissues are visible in the CBCT images. The image stability and reconstructed precision is satisfying.