The present work addresses the experimental evaluation of fatigue properties of APIX65 steel. Specifically, da/dN vs ΔK curves and fatigue threshold values (ΔKth) are discussed. Using C(T) specimens, Fatigue Crack Growth (FCG) tests were performed at R = 0.1 and R = 0.7 in air and at room temperature, according to E467 ASTM Standard. Data from these tests are then treated and analyzed. For all specimens, da/dN vs ΔK curves are plotted and discussed. MATLAB routines are developed to calculate the da/dN rates using raw data of the FCG test machine (MTS Flex Test 40). Paris Law coefficients are obtained and compared to literature values of similar API steels. A fatigue threshold value was obtained at R = 0.7. The influence of load ratio (R) is also analyzed. Finally, a comparison and discussion of all the results obtained in this work is done with values found on other studies.