The effect of low temperature magnetic field annealing on the soft magnetic properties of Fe_(81-x)Co_xB_(14)C₁Si₄(x=0-12) amorphous alloys have been investigated. The crystallization and Curie temperature are not changed with the Co content. By low temperature magnetic field annealing, the magnetic flux density increases and the coercive force and magnetostriction decreases. These effects are discussed in terms of the structural relaxation and the unidirectional induced magnetic anisotropy. The maximum permeability has a inverse relation with magnetostriction. The best magnetic property are obtained by the magnetic field annealing at 375℃ for 2 hours in Fe_(69)Co_(12)B_(14)C₁Si₄amorphous alloy.