The English legal system is representative of the common law tradition. In England and Wales, the modern public prosecution system was only created in the last quarter of the 20th century which was very recent when compared to other countries. This study aims to explore why the English system of criminal justice established the Crown Prosecution Service (CPS). The CPS was created by the Prosecution of Offences Act 1985. This set up an independent public prosecution system in order to improve efficiency and protect the interests of the defendants by separating the prosecution from investigation and preserving the filtering role of the prosecutors. Before the establishment of the CPS, the police in fact exercise the prosecution. However, the prosecution by the police created an unclear border between the investigation and prosecution. Such a functional mixture caused problems such as the prosecutions of evidentiary weak cases or ‘overcharging’ because police involvement restricted the prosecutor’s role as a filter to screen the results of the investigation. The development of the CPS shows the direction to which the Korean system of criminal justice should flow. Korean prosecutor’s direct involvement in the investigation can cause problems which were already observed in the English system as it leads to the functional mixture of investigation and prosecution. The roles and powers of the prosecution service must be reformed to recover their role as a significant filter in screening the result of investigation.