An electrochemical ferro sensor with ZrO2(MgO), MgFe2O4 and Mo + MoO2 as the solid electrolyte, auxiliary electrode and the reference has been developed. The calculating equations of activity, activity coefficient and concentration of ferro in nickel matte were obtained from experimental data. The sensitivity of sensor was also studied in the paper. The sensor was submitted to control the concentrations of ferro in matte at the end-point of the blow in converter process. During such in-plant tests, it was confirmed that the stability of the sensor is excellent and it facilitates more accurate control of ferro contents at the end-point of nickel matte blowing.