This research includes experimental investigation and theoretical simulation which is manufactured locally in a laboratory to assess the characteristics of the two-phase convective heat transfer and performance of a loop thermosiphon. Practical experiments were performed using pure water, hydrocarbon fluid (acetone) and a mixture of both fluids with determined weight ratios used as working fluids. The experimental results of average heat transfer coefficient that were theoretically simulated under the same boundary conditions in both cases to study the effect of the kind of working fluid on the period required to reach the steady state condition and to achieve high performance within the best design conditions of the experimental rig. A comparison was made between the simulation and experimental results and an acceptable agreement was shown. It is worth mentioning that the Computational Fluid Dynamic analysis (CFD) with Engineering Equations Solver (EES) were used in the theoretical simulation.