The paper presents the results of analysis, development and tests of an ammonia miniature loop heat pipe (MLHP) with a flat evaporator, which has an active-zone diameter of 30mm. The length and the diameter of the vapor and the liquid lines are 1m and 2/1.2mm. The device serviceability has been demonstrated at a horizontal and a vertical orientation in 1-g conditions. The maximum heat load achieved on trials was equal, respectively, to 160W and 120W, which corresponds to a heat flow in the evaporation zone of 23 W/cm2 and 17 W/cm2. The minimum thermal resistance at nominal heat loads from 40 to 80 W varied in the range from 0.42 W/m2K to 0.59 W/m2K. A comparison has been made with a model MLHP with a cylindrical evaporator equipped with a copper and aluminum “saddle”.