Scanning Maxwell stress Microscopy (SMM), which is a type of scanning probe microscopy designed to image microscopic electrical properties, has been applied to cyanine dye Langmuir-Blodgett (LB) monolayers. In combination of epifluorescence microscope technique with SMM, the cyanine J-aggregate formation way was clearly observed. Further the light induced electron transfer between cyanine and viologen in LB film system could be detected as a surface potential change by SMM technique.