The paper presents the results of the research on a series of reinforced concrete beams strengthened with the new Polish CFRP post-tensioning system with various prestressing levels (force). The tests included four RC beams with a rectangular cross-section of 0.50 m x 0.42 m and a length of 6.0 m. The first beam was a reference (control) beam. The next three beams were strengthened by prestressing with two CFRP strips. Prestressing level is defined as a ratio of prestressing force to the characteristic ultimate tensile strength of CFRP strip. Tested beams were prestressed with 30%, 40% and 50% of CFRP tensile strength which corresponds to a force of 162 kN, 216 kN and 268 kN for subsequent beams. The beams were investigated in a full range of flexural behaviour, including post-debonding phase. The results indicate that changing of prestressing level has a significant influence on behaviour of prestressed beams. The beams strengthened with higher prestressing level exhibited a higher cracking, and steel-yielding moments as well as slightly higher level of CFRP utilisation. Changing the strip prestressing level did not affect the load-bearing capacity of the beams. Beams with different prestressing levels exhibited similar flexural behaviour until strip debonding. Differences occurred in post-debonding phase of work. The higher prestressing level, the failure mode of the beams was more sudden. On the basis of the failure mode of the tested beams, the optimal prestressing level of the CFRP strip in the new prestressing system was determined.