I found a way to do it by exporting the pixels data of each band using the function pixel to ASCII under the tab of manage data in ERDAS. You need each band in a separate file, Then in the dialog of pixels to ASCII you just click add (bands). Open the two bands (exported) in EXCEL and use scatterplot to derive a linear relationship. The method does not take much time in preparing data in EXCEL as the two bands will be in two different columns. NO direct method in ERDAS.