Abstract: Phages purification is the basic work to study the characteristics and analyze the genome.A method to purify Streptococcus thermophilus phages from culture medium by means of CsCl gradient centrifugation was described.The main processing included polyethylene glycol ( PEG) concentration, dialysising to remove the PEG, CsCl density gradient centrifugation and dialysising to remove the CsCl. Multiple polymerase chain reaction and Sodium dodecyl sulfate polyacrylamide gel electrophoresis showed that the packaging mechanism of the phage was pac-type.