Article 38 of the Statute of the International Court of Justice(ICJ) is considered to be the provider of a list of reliable sources for international law. Paragraph (1) (b) of the Article introduces customary law as the general practice of States members of the international community recognized as law(opinio juris). Therefore, each argument regarding customary international law as a main formal source, should be in line with the requirements of the mentioned Article. Although, before the Statute became binding, the concepts stated in this Article were already enforceable, the concepts may not have the same meaning in modern international criminal law whose main objective to prosecute and punish individual criminals committing humanitarian and human rights violations. The main objective of the present article is to demonstrate the extent the requirements of the establishment of custom in Article 38, i.e State practice and opinio juris, is applicable to criminal customary law. To do this the statutes of international criminal tribunals for former Yugoslavia(ICTY) and Rwanda(ICTR), human rights documents and the case law of the international criminal tribunals are examined. It seems that Article 38 of the Statute cannot be strictly applied to international customary law in criminal cases particularly when it comes to the application of the principle of legality or nullum crimen, nulla poena sine praevia lege poenali as a defence in criminal prosecutions.