An experimental investigation of the thermal performance of a flat plate heat pipe is presented in this work. The results indicate that the temperature along the heat pipe wall surfaces is quite uniform. The results also indicate that the porous wick of the evaporator section creates the main thermal resistance resulting in the largest temperature drop, which consequently affects the performance of the heat pipe. The idea of the heat pipe time constant is introduced in this work to describe the transient characteristics of the flat plate heat pipe and an empirical correlation for the time constant in terms of input heat flux is presented. Correlations for the maximum temperature rise and maximum temperature difference within the heat pipe are also presented. The experimental results at steady state were compared with the analytical results and found to be in good agreement. This work constitutes the first detailed experimental investigation of a flat plate heat pipe.