Proceeding of the Joint International Symposium on Deformatuon Monitoring. 2-4 November, 2011. Hong Kong. China. Report. The incoming restoration work of Duomo di Milano cathedral main spire requires a structural health monitoring. A weight of 90 t due to scaffoldings was applied to the dome and a complex monitoring system was designed in order to measure the deformation of the church and its spire. The paper gives a presentation of the different methods used to investigate the different structural components including: optical leveling, robotic total stations, accelerometers, strain gauges, optical plumbs, clinometre, and extensometre. In particular for the analysis of structural elements at higher risk of collapse, fiber optic sensors were selected, based on FBG (Fiber Bragg Grating) technology. Strain of the lower part of the vaulting-rigs inside the octagonal doom is the measurement of interest. As the expected signals are very small and the thermal disturbances very significant, a thermal characterization of two types of commercial strain gauges was carried out in laboratory with a thermal chamber. A block of the same marble used for the cathedral was tested. This allowed to find a relationship to be used later to compensate for any thermal effects. Finally, some sensors are placed at critical points to record real time deformation measurements. A central control unit will host an alarm system and GUI to show all results.