We newly calculated the line radiative force with 520,000 atomic lines, which is twice as many as those of Abbott (1982), for OB supergiants. Our results are as follows. (1) The mass loss rates for O stars with T
eff = 50,000K are seven times as large as Abbott's (1982) because of contribution from Fe iv lines. (2) Contribution from many weak lines increases the mass loss rates and decreases the wind velocities of OB stars within a temperature range of 10,000K ≤ T
eff ≤ 30,000K. This result is qualitatively in accordance with the results from the recent observations of O stars. (3) The mass loss rates of OB stars depend on metallicity with Ṁ ∼ Z.