This paper shows a theoretical model for reinforced concrete beams subjected to torsion and strengthened with composite of carbon fibers (CFRP). It is based on researches of Michael Collins and collaborators and approaches the theory of diagonal compressive field. This model was inserted in a mathematic software, due to the need of interactions to obtain the torsion and torsion angles. The validation of theoretical values was realized through the results of tests obtained from the literature and it was understood that the model is valid and conservative for the ultimate torsion in most of the studies carried out. However, the beams that had the geometric ratios width and height b/h < 0.5 and the ratio between the cover and the smaller dimension of the d/cob element ≤6 had not so accurate results.