A thermocompressor is the equipment which compresses a vapor to a desired discharge pressure. Since it was first used as the evacuation pump for a surface condenser, it has been widely adopted for energy saving systems due to its high working confidence. In the present study, the geometrical analysis of the shape between the jet nozzle and the diffuser inlet, the drag force was calculated by means of the integrated equation of motion and the computational fluid dynamic (CFD) package called FLUENT. The computer simulations were performed to investigate the effects by the various suction flow rates, the distance from jet nozzle outlet to the diffuser inlet and the dimensions of the diffuser inlet section through the iterative calculation. In addition, the results from the CFD analysis on the thermocompressor and the experiments were compared for the verification of the CFD results. In the case of a jet nozzle, the results from the CFD analysis showed a good agreement with the experimental results. Furthermore, in this study, a special attention was paid on the performance of the thermocompressor by varying the diffuser convergence angle of $0.0^{circ}$, $0.5^{circ}$, $1.0^{circ}$, $2.0^{circ}$, $3.5^{circ}$ and $4.5^{circ}$. With the increase of the diffuser convergence angle. the suction capacity was improved up to the degree of $1.0^{circ}$ while it was decreased over the degree of $1.0^{circ}$.