A very compact air-cooled loop thermosyphon cooling system (LTS) was designed, prototyped and tested for microprocessor cooling application. It was designed specifically for 2U servers and heat loads up to 400 W in a footprint area of 40 mm per 40 mm. The low pressure and low GWP working fluid R1233zd(E) was used. Tests were done for two ambient temperatures (22 °C and 40 °C) and included optimal charge determination as well as extensive tests at optimal charge. Values of performance ratio, simply defined as heat load divided by fan power consumption, higher than 30 were observed for the maximum heat load of 400 W. The experimental results were also used to validate JJ Cooling Innovation’s inhouse proprietary solver developed to design the LTS and results will be presented.