The S-K-K peak in Fe-P alloys has been studied by a vacuum Kê-pendulum. It is found that phosphorus of substitutional solid solute element strongly reduces the peak height (Qh-1), and obviously lowers the peak temperature (Tp) as well as evidently decreases the activation energy (H) of relaxation; the I-F peak height Qh-1 reduces linear-ly with an increase in phosphorus concentration to two thirds power Cp2/3 (namely thesquare of the phosphorus atom concentration on dislocation lines) when the concentration of phosphorus is below its solid solute limit in iron. The experimental results are ccnsitent with Sehoeck's theory. The mechanism of the I-F is also discussed and is attributed to the dragging by pinned non-screw (including edge) dislocation segments of a Cottrell atmosphere of interstitial solute atoms, by migration of the solute atoms and the dislocation segments moving by bowing.