In this work,the influence of strain on threshold energy of absorption in Silicon circular nanowires is investigated.For this purpose,we first have used the density functional theory(DFT) to calculate the electron and hole effective masses.Then,we have obtained absorption threshold energy with two different procedures,DFT and effective mass approximation(EMA).We have also obtained the band structures of Si nanowires both DFT and EMA.The results show that:i) the expansive strain increases the hole effective mass while compressive strain increases the electron effective mass,ii) the electron and hole effective masses reduce with decreasing the wire size,iii) the absorption threshold energy decreases by increasing strain for compressive and tensile strain and its behavior as a function of strain is approximately parabolic,iv) the absorption threshold energy(for all sizes) obtained using EMA is greater than the DFT results. In this work, the influence of strain on threshold energy of absorption in Silicon circular nanowires is investigated. For this purpose, we first have used the density functional theory (DFT) to calculate the electron and hole effective masses. threshold energy with two different procedures, DFT and effective mass approximation (EMA) .We have also obtained the band structures of Si nanowires both DFT and EMA.The results show that: i) the expansive strain increases the hole effective mass while compressive strain increases the electron effective mass, ii) the electron and hole effective masses reduce with decreasing the wire size, iii) the absorption threshold energy decreases with increasing strain for compressive and tensile strain and its behavior as a function of strain is approximately parabolic, iv) the absorption threshold energy (for all sizes) obtained using EMA is greater than the DFT results.