Investigation of electronegative plasmas has now been atrractive due to the advantages of negative-ion assisted etching and charge-free ion implantation in semiconductor manufacture. Langmuir electrostatic probe, as a simple, inexpensive and good spatial resolution diagnosic tool, is popularly used in investigating electronegative plasmas. In this paper, the Langmuir electrostatic probe is proposed to measure the capacitively coupled Ar plasmas with added electronegative gases, such as O2, Cl2 and SF6. The experimental results from the measurements of Ar plasmas with added electronegative gases driven by a 40.68 MHz field indicate that, with increasing flow rate of electronegative gas, high energy peak will occur in electron energy possibility function and shift towards higher energyside. The addition of electronegative gases reduces the electron density significantly as the electron temperature increases. We also calculate the electronegativity of Ar plasmas for the three kinds of electronegative gases. The preliminary interpretations of the above experimental phenomena are presented.