Abstract: Since the successful launch of various kinds of remote sensing satellites,there are many kinds of remote sensing satellite images available.Information extraction from those remote sensing data sources usually requires to analyze those data sets together,and to visualize on a media such as paper,photo,monitor,etc.Image fusion is a technique to do so.This paper traces the processes of image formation,analyses the basic physical theory about remote sensing image fusion in frequency domain,and provides the frequency domain basis on how to design a proper image fusion algorithm.