A fast and efficient method for analyzing an inset dielectric guide is presented using the Fourier transform technique with a modified perfectly matched boundary. In order to deal with an open region, a novel idea, modified perfectly matched boundary condition (PMB), has been proposed. By introducing the modified PMB, the numerical integral has been avoided and the accuracy of the numerical solution has been improved. Moreover, the singular behavior of the fields at metal edge is taken into account in the analysis. The numerical examples are shown that the convergence of the solution is very fast and the relative error less than 0.07% is attained even if only the first term is considered in the field expansion of the guide. The numerical results of the propagation constants for single- and double-layered inset dielectric guides agree well with those of literatures.