Conventions that include private law norms play a special role among international agreements. Their objective is to streamline rules concerning a specific legal measure. Convention on Contracts for the International Sale of Goods (CISG) and conventions on contracts of carriage of goods in individual branches of transport have such a character. Provisions of these conventions regulate legal relationships between entities under private law. They are applied directly by national courts if a given convention is applicable to a given legal relationship. Adopting a convention and bringing it into force does not end the process of unifying the law. Law is truly uniform if it is uniformly applied by national courts in individual countries which are parties to conventions. However, it is difficult to achieve. The aim of this paper is to demonstrate obstacles in courts uniform application of conventions that comprise private law norms and negative effects of non-uniform application of such conventions. Methods that can serve to strengthen the unity of application of conventions are also specified. Appointment of advisory bodies which issue non binding opinions on the interpretation of provisions of a given convention is a plausibly effective solution when certain conditions are met. The analysis of legal acts and of judicial decisions of national courts from various countries and the European Union are the research methods applied by the author. Literature studies have also been carried out in order to obtain the broadest possible picture of the discussed issue.