This study is the report of modelling the evacuated tube solar collector (ETSC) by Gene-Expression Programming (GEP). The data were gathered by simulating the ETSC for different volumes of the thermal storage tanks (10-50 Lit) with various solar radiation intensities through Computational Fluid Dynamics (CFD). In order to obtain the most accurate mathematical model (expression) and better predicting the efficiency of system, a trial and error approach was employed. The GEP model was tested and trained based on the numerical data. On the other hand, the numerical data were verified by comparing the results with the previous experimental investigations. The numerical results indicated that as the solar radiation absorption increases, the water temperature difference and the energy efficiency of the ETSC also increase. Moreover, for the ETSC possessing three evacuated tubes, the optimum volume of the thermal storage tank was obtained 26 Lit, and the maximum energy efficiency of the optimized ETSC for the numerical investigations was reported roughly 72%. In the previous published work, the maximum amount of efficiency of the ETSC was 70% showing that the CFD-based approach could acceptably model the performance of ETSCs, and it can be concluded that the GEP-based model for the ETSC performance is reliable and trustworthy.