In this study, electron density is extracted from I-? curve of single OML probe. This analysis is committed by using the ionic part of the curve only. The calculations run for many curves based on the data were for UMIST quadrupole. It is found a great discrepancy between the ion density (plasma density) and the extracted electron density. Our explanation for this phenomenon is the effect of the wall of the vessel, which can act as a second probe with OML single probe. Accordingly the calculated electron density does not represent plasma density, but the affected density. This effect causes a sort of deviation on plasma potential. Then this deviation appears clearly in sheath potential ratio.