This paper deals with an investigative document (WathiqatKashf). The author found this document in the archive of Bab Al-Aali Court as The document is very important for studying the mausoleum of Qala'un through Ottoman period where the original dome of Qala'un mausoleum was described.. This description presents new information about materials and method of the construction, which have been used in building of this dome. According to this document, the original dome was built of wood covered with plates of lead. Depending on the text of this document as well as other resources, the author started to follow up the history of Qala'un dome through the Ottoman period. This study revealed new several facts and corrected some information of previous studies related to the architectural history of the dome through the Ottoman period. This study also deals with the process of rebuilding of the wooden dome and repairing the original wooden screen Mashrabiya (Maqsura) and the coffin (Tarkeba). In addition, the study followed the basics, which had been adopted to carry out these artworks. The author interprets also the role of the court to follow the steps of performance of these works. The author is very interested to show the role of the technical treatments done by architects and craftsmen in rebuilding the wooden dome and repairing the coffin and Maqsura. The document contained many various architectural terms. The study deals briefly with those terms and their meaning through Mamluk and Ottoman period. The notary of this document used some terms that differ from those have been used through the Mamluk period, especially the terms related to the functions of Qala'un complex. The author interprets the reasons of these changes. Results of this study show the fault of project of rebuilding the dome of Qala'un mausoleum, which had been carried in 1903 and show the importance of the investigative documents in preparing of the conservation projects of Islamic monuments.