An analytical formula for the additional beta due to fast fusion-bo ions is derived by using the slowing-down approximation from the Fokker-Planck equation under the assumption of negligible loss term. It is found that the fast ion beta in a D-3 He fusion plasma at a typical temperature of 55 ke V is about 20% of the thermal beta,which is the same ratio as that obtained in a D-T plasma at 20keV.