<p>Francis turbines suffer dangerous pressure fluctuations at partial load, which must be prevented. In this article, the effect of different free and forced aeration methods were investigated to reduce pressure fluctuations. Air was introduced into the low-pressure areas of the draft tube in a free and forced methods using two locations, one through the center of the runner cone and the other through a special structure placed at different locations of draft tube. Governing two-phase flow equations were numerically solved using SAS-SST turbulence model for simulation. The results show that free aeration method using special structure located at lower section of the draft tube has the highest reduction in pressure fluctuation amplitude by 40%. Also, comparison of forced aeration cases showed that the greatest decrease in pressure fluctuations occurs using aeration with 3% of the total flow rate through the special structure located at lower section of the draft tube with a value of 36%. Finally, the optimization was done by considering different cases of free and forced aeration by defining performance index (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\Gamma \)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="normal">Γ</mi> </math></EquationSource> </InlineEquation>) using 4 important parameters. The result showed that free aeration from the structure located at lower section of draft tube has the best performance.</p>