Favor contractus principle means that in contract law the priority would be given for the existence, validity and execution of contract. This principle was moved to the Civil Code of the Republic of Lithuania after the integration of almost the whole UNIDROIT principles system of international commercial contracts. Neither of the documents entrenches the concept of principle favor contractus, as such, nor regulates it. Principle favor contractus is evaluated systematically by analyzing the legislative norms and is interpreted regarding various aspects of legislative norms. The aims of favor contractus principle may be defined as the attempts to reduce the number of cases when the existence and validity of contract could be questioned or the contract could be terminated in advance as much as possible, therefore this principle as such is necessary. The main fields of contract law where the key elements of principle favor contractus could be distinguished are the institutes of contract conclusion, invalidity and execution. The first part of the thesis reviews the principles of contract law – contractual rights, pacta sunt servanda, consensualism, honesty, favor contractus, the weak contract party’s defense in contractual relations – as well as its content entrenched in international documents, legislation of the Republic of Lithuania. The second part analyses the principle favor contractus, its concept is presented and the content of principle elements is determined. The third part summarizes the Lithuanian judicial practice by applying favor contractus principle in separate institutes of contract law. In Lithuanian courts of lower instances this principle is not perceived properly, because there are cases when the contract making rules, submitting modified acceptance, are ignored. Moreover, it is avoided to apply alternative methods of assertion for the contract to be recognized as invalid, it is also rushed to terminate the contract without seeing whether the breach is really essential.