The results of heat transfer and hydrodynamics investigation in the time of water vapor condensation in a narrow gap condenser of a loop heat pipes. The length of the condenser is 80 mm. A cross-section is 40x1.1 mm. Instrumental investigations and visual observations of a condensation process of a condenser with longitudinal guide insertions and without them was carried out. The investigations were conducted at the condenser cooling temperature varied from 60 to 95 °C. At all operating parameters there was a stratified flow regime of a two-phase flow and a film type of condensation. A temperature field in the condenser and the coefficient of heat transfer were measured. Heat transfer coefficients for the condenser without the insertions are in a range of 21 to 46 kW/(K-m2), and with the insertions - from 42 to 88 kW/(K-m2).