The performance of a thermosiphon based parabolic trough collector (PTC) used for direct steam generation depends largely on the heat losses of the solar thermal system. This paper presents an experimental investigation of the heat losses in a thermosiphon based solar thermal system that used a linear receiver with a PTC for the generation of low temperature steam. A locally constructed PTC was used to concentrate sun rays to a linear copper pipe enclosed in an evacuated glass tube and held at the focal line of the PTC to heat water and generate steam. Circulation of the water in the closed-loop solar thermal system was through natural convection. A solar meter was used to measure the incident radiation flux at the experimental site and PT100 temperature sensors were installed at different points of the system to measure the temperature distribution within the system. The thermal efficiency and overall heat losses of the system were investigated by fitting the experimental data to standard equations. The results showed that the instantaneous thermal efficiency of the system was 46.48%, 43.1% and 45.32% respectively for three days examined. The overall heat losses in the system were 1211.95, 974.32 and 911.26 kwh per day respectively for the three days investigated. Heat losses from the tank accounted for over 83% of the losses for all the days examined. The evacuated glass tube reduced heat losses from the receiver to very low values of 2.31, 1.63 and 1.43 KWh per day respectively for the three days tested. The use of a better insulating material on the tank was recommended to reduce convective and conductive heat losses, thereby enhancing the performance of the system.