This paper is based on the material on the judicial oath formulas on the 14th and 15th centuries from the area of central Lesser Poland (Krakowskie region) and northern Lesser Poland (Sandomierskie, Chęcińskie, Radomskie regions). The thesis material was excerpted for the purpose of analysis selected linguistic (mainly phonetic) characteristics of judicial oath formulas from Lesser Poland and finding an answer to the questions about the pronunciation characteristics of that province in the 14th and 15th centuries as well as about internal diversification of the Lesser Polish dialect. On this basis, it was possible to identify dialectal differences at three levels: characteristics differentiating Lesser Poland from other provinces, differences between central and northern Lesser Poland, and finally dialectal pronunciation diversities among people living in northern Lesser Poland - lands only: Sandomierska, Chęcińska and Radomska.