We present a simple method to extract the effective doping concentration related to the LDD (Lightly Doped Drain) regions in fully depleted SOI MOSFETs. The series resistance of an LDD structure MOSFET is composed of different components, the LDD series resistance, being the dominant one. The proposed method uses the back gate voltage influence on the back interface below the LDD region. MEDICI simulations were used to support the analysis. Experimental results obtained from I-V data were compared to the simulated results demonstrating a good agreement.