Spinodal　decomposition　in　Cu-4Ti　has　been　studied　by　transmission　electron　microscope　(TEM).　It　is　indicated　that　spinodal　decomposition,　which　is　not　supressed　by　rapid　quenching　(brine　cooling),　is　an　elementary　phase　transformation　process　in　Cu-Ti　alloys.　The　wavelength　of　the　modulated　as-quenched　structure　is　approximately　5.7nm.　A　new　idea　for　the　development　of　the　modulated　structure　has　been　offered,　that　is,　it　takes　place　in　the　two　stages:　spinodal　decomposition　and　growth　of　the　modulated　structure.　The　law　λ3　-　λ30　=　Kt　holds　for　two　stages.　The　growing　activation　energy　of　mean　modulation　wavelength　for　the　above　stated　two　stages　are　176　kJ/　mol　and　134　kJ/　mol,　respectively.