Every conservation works related to an ancient masonry structures should be preceded by an appropriate diagnostic. This should be understood as geometrical survey and various tests, which results with a proper analysis of the structure, identification of materials, technologies and techniques used during construction. The effective tool which could be used in this field is 3-D laser scanning. The digital image obtained as a result of scanning could be a proper base for a preservation programme, as well as help for creation of a precise digital models for a structural analysis. The examples of 3-D laser scanning application presented in the article are diagnostic works carried by the Division of Fundamentals of Building, Warsaw University of Technology, with the cooperation of Warsaw University, at the archaeological sites in Alexandria, Egypt and in Tanais, Rostov, Russia. Based on this works some most important advantages of laser scanning in identification, diagnostics and preservation of ancient architectural monuments was stated.