On 11th of October 2011 The European Commission (EC) announced Proposal for a Regulation of the European Parliament and of the Council on a Common European Sales Law (CESL). The presented proposal is a result of works conducted towards harmonization or unification of contract law regime within the scope of the European Union. EC stipulates in the submitted documents that the main aim of the project is to improve establishment and functioning of the internal market by facilitating expansion of cross-border trade for business and cross-border purchases for consumers. The whole project, which subject matter is harmonization of European contract law by establishing a second regime of contract law in each member state caused many doubts and objections. The latter regulation on CESL presented by the EC is neither free of doubts, namely the scope of the regulation (private law, contract law, tort law, etc.), the subject matter of the regulation (business-to-business and business-to-consumer contracts), form and optional character of the regulation. Irrespectively of those issues, the main problem regarding project presented by the EC seems to be the legal (treaty) basis for the future regulation on CESL. According to the proposal of the EC the indicated legal basis is art. 114 of the Treaty on the Functioning of the European Union, which regards harmonization of law. The possible inconsistency of the regulation on CESL with mentioned article of the Treaty could cause invalidity of the whole regulation. Moreover, above mentioned objections also concern the necessary observance in relation to the two general principles of EU, namely subsidiarity and proportionality principles. All the issues mentioned above as well as doubts and objections arising from the CESL project are the subject matter of this article.