In this paper, the expression for the double probe V-l characteristics and a formula to determine the electron temperature are derived. They are applicable whether the R. F. disturbance voltage drop across the probe sheath exists or not. The influence of this R. F. disturbance voltage on the double probe diagnostics for a low pressure plasma is investigated. Furthermore, the electron temperatures of the R. F. plasma under the pressure of 10-2—10-3 Torr are obtained experimentally. Experimental results agree with the theoretical analysis very well, k is also demonstrated that, if the electron energy distribution is Maxwillian, the R. F. disturbance has no substantial effect on the determination of electron temperatures of the plasma.