The results of developing and investigating the operating characteristics of a copper-water loop heat pipe with an effective length of 330 mm are presented. The pipe was equipped with a flat-oval evaporator 7 mm thick with an active zone measuring 30x30 mm and a condenser with a spiral finning and a cooling jacket. The tests were conducted at horizontal and vertical orientations of the device in the gravity field in a heat load rage from 5 to 1400 W at water temperature, cooling a condenser, varying in the range from 10 to 80 °C. The dependence of parameters, characterizing the operating efficiency of a LHP, was studied in tests. The research on a start-up of the device at low and moderate heat loads was also carried out.