A modification to the conventional coarse mesh diffusion theory was made at M.I.T. and applied to Fast Reactor Analysis. The present study uses basically the same method which is coded into the DIFDIS(DIFfusion equation with flux DIScontinuity factors). Unfortunately, when applied to the realistic PWR``s, the large discrepancy in the predicted power distribution was found. It is attributed to the facts that the shape of the thermal flux in the vicinity of the core boundary does not share the shape of the fast flux, and that both thermal and fast flux gradients are very steep at the core boundary. These facts wholly result from the existence of the baffle. Two methods are developed from this study which treat the influence of the baffle effectively. They are the Boundary Color Set Method and the Global Local Iteration Method which are different from the current methods in the fact that in both methods the local calculations are done for only core boundary nodes. Both methods give excellent results, and need the small core memory and computing time which make the modified nodal diffusion method competitive with the other modern nodal methods in the viewpoint of economy and accuracy.