De-noising algorithms for volume CT are proposed based on the correlation of the cone beam (CB) CT projections. The main idea is: when the correlated pixels among the neighboring views are added together according to this projection correlation (PC), the signal will be retained, whereas the noise variance upon them will be reduced by this averaging process, because the signal obeys this correlation but noise not. PC based filtering (PCF) method is developed. PCF is further integrated as PC based penalty weighted least square (PC-PWLS) method. Validations are performed for two volume scans: one is ideal low-dose case, and the other considers the high-frequency scatter residues after scatter correction. By using the proposed methods, significant noise suppressing is illustrated; meanwhile, the spatial resolution can be well preserved. For the low dose scan, SNR (Signal to Noise Ratio) increases are around 14 dB. For the cone beam scan with high frequency scatter residues, an effect approaching to the fan-beam scan is achieved.