This paper deals with the research of wall paintings in the underground of today's Dubrovnik Cathedral, focusing on a pre-Romanesque building discovered in the 1980s. In the introduction history of the cathedral is explained shortly. Works done on archeological site, together with the found remains of historic buildings that preceded the baroque cathedral, are shown. Additionally, many of the fragments of wall paintings, ceramics, and stone elements were found on the same archeological site. The process of their re-examination and inventory began in 2015. The process is a part of the student workshops "Discovery of the old Dubrovnik cathedrals "of the The Faculty of Humanities and Social Sciencesin Zagreb in cooperation with the city Parish of Our Lady the Great in Dubrovnik. The students of the Department for conservation and restoration of art joined the workshop in 2018. During these research and educational workshops, the main activity was working with fragments of wall paintings. The fragments were scanned, and the most common plaster types are recorded, listed and described. A new way of adequate fragment storage was agreed upon and conducted. For each storage box, a document was opened in which previously scanned fragments are marked and divided by types of plaster. This is done to ease data entry in the database. In addition to the work on the fragments, great attention was paid to the findings in situ in the cathedral's underground. The plasters on the fragments were compared and connected with the remained ones of archeological buildings sites. Drawing on the south wall of the oldest church with a scene of Baptism, and examples of graffiti were documented. The most common plasters from this building are described and the analysis results of samples of these plasters are presented. The process of dating with radioactive carbon by the accelerator mass spectrometry (AMS 14C) technique has been specially processed. This thesis presents the as yet unpublished results of the analyses by the Civil Engineering Institute of Slovenia laboratory and results of plaster dating performed at the Institute Ruđer Bošković.