A new iteratively equivalent index method for the analysis of rectangular dielectric waveguide problems is presented. This analysis is found to be more accurate than the effective index method and the modified effective index method. The results obtained by this method compare very favourably with those produced by the variational analysis, weighted index analysis and numerical computation over a large range, but without the complexity of the latter. A simple analytical expression is also derived to describe the field distribution of such a waveguide from two equivalent coupled slab waveguides at right angles.