An accurate way of determining the series resistance R(s) of Schottky Barrier Diodes (SBDs) with and without the interfacial oxide layer using forward current-voltage (I-V) characteristics is discussed both theoretically and experimentally by taking into account the applied voltage drop across the interfacial layer V-i. For the experimental discussion, the forward bias I-V characteristics of the SBDs with and without the oxide layer fabricated by LEC (the Liquid-Encapsulated Czochralski) GaAs were performed. The SBD without the oxide layer was fabricated to confirm a novel calculation method. For the theoretical discussion, an expression of V-i was obtained by considering effects of the layer thickness and the interface state density parameters on forward bias I-V of the SBDs. The Value R(s) of the SBD with interfacial oxide layer was seen to be larger than that of the SBD without the interfacial oxide layer due to contribution of this layer to the series resistance. According to the obtained theoretical formula, the value of V-i for the SBD with the oxide layer was calculated and it was subtracted from the applied voltage values V and then the value of R(s) was recalculated. Thus, it has been shown that this new value of R(s) is in much closer agreement with that determined for the SBD without the oxide layer as predicted. Furthermore, the curves of the interface states energy distribution of each sample are determined. It was concluded that the shape of the density distribution curve and order of magnitude of the density of the interface states in the considered energy range are in close agreement with those obtained by others for Au/n-GaAs Schottky diodes by Schottky capacitance spectroscopy.