Development of high bit rate services requires extension of the telecom network and then the power inside the France Telecom telecommunication outdoor cabinets. Traditional cooling systems through air forced convection become inefficient for high power supply. For this reason, thermosyphon loop is developed for cooling telecommunication equipments in the outdoor cabinet in order to keep equipments temperature below the operating temperature defined by European Telecommunication Standard Institute (ETSI). In this paper, experimental investigations using a France Telecom telecommunication outdoor cabinet fitted with thermosyphon loop are presented. Transient and steady states analysis of the thermosyphon loop efficiency, the temperatures distributions, the thermal resistance, and mass flow rate and heat losses by convection in the walls of the cabinet as a function of heat load are studied. The n-pentane is used as the working fluid. Moreover, different working fluid filling ratios are tested and the results show that the optimal filling ratio is about 9.2%. This value corresponds to the minimum of operating temperature and also the minimum of system thermal resistance. In order to meet the ETSI norm, the traditional cooling system (air convection) currently used by France Telecom and the thermosyphon loop cooling system are compared. The results show that the maximum heat load of the telecommunication equipment obtained with the thermosyphon loop cooling system is twice as important than that given by, the traditional cooling system.