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Based on the Fourier–Chebyshev spectral method, the control of turbulent channel fl ow by space-dependent electro-magnetic force and the mechanism of drag reduction are investigated with direct numerical simulation (DNS) methods for different Reynolds numbers. A formula is derived to express the relation between fl uctuating velocities and the friction drag coefficient. With the application of electromagnetic force, the in-depth relations among the fl uctuating velocities near the wall, Reynolds stress, and the effect of drag reduction for different Reynolds numbers are discussed. The results indicate that the maximum drag reductions can be obtained with an optimal combination of parameters for each case of different Reynolds numbers. The fl uctuating velocities along the streamwise and normal directions are suppressed significantly, while the fl uctuating velocity along the spanwise direction is enhanced dramatically due to the spanwise electromagnetic force. However, the values of Reynolds stress depend on the fl uctuating velocities along the streamwise and normal direc-tions rather than that along the spanwise direction. Therefore, the significant effect of drag reduction is obtained. Moreover, the maximum drag reduction is weakened due to the decay of control effect for fl uctuating velocities as the Reynolds num-ber increases.