The first part of the paper contains a general discussion on the principle of legality and the principle of opportunity in criminal prosecution, including definitions of terms as well as explanations of meaning, scope of application and relation between these principles. Further, the author analyzes all cases of discretionary criminal prosecution in juvenile criminal procedure proscribed by the Law. Although the principle of legality is fundamental to the criminal prosecution in juvenile criminal procedure, the number of proscribed exceptions from this principle is constantly increasing, which contributes to the extension of scope of validity of the principle of discretionary prosecution. The paper initially examines legal conditions for application of the principle of discretionary prosecution prior to commencement of juvenile criminal proceedings: in the cases of minor criminal offences, when juvenile delinquent is serving imprisonment sentence and in the case of conditional commencement of the proceeding. The following analysis relate to the application of discretionary criminal prosecution in an ongoing proceeding. In the author's view, these are specific legal grounds for termination of proceedings. For these reasons, by author's interpretation, the proceedings are terminated either for on the basis expediency or conditionally.