Ocean thermal energy conversion (OTEC) uses the seawater temperature difference between the surface and the deep to generate electricity. In the process, the optimum running condition of the heat source and heat engine gives the maximum net power. Conventional thermal efficiency and exergy hardly express the effectiveness of the system, although the system is fairly simple compare to the other complex chemical processes. This paper evaluates the comprehensive power generation performance by applying the finite-time thermodynamics (FTT) models, which is able to analyze the performance of OTEC systems. The system applied the Rankine cycle with the ammonia as the working fluid using cross-flow plates as the evaporator and the condenser. The FTT model based on the experimental are constructed. As the results, the analysis and the model reveals the power generation characteristic and the overall internal irreversibility of the heat engine.