There are many Hangul input systems to input Korean letter on computer or smart phone. However, the existing systems need to be required more efforts to modify the already inputted letters. This research suggests the Hangul letter modification method which can modify letter effectively based on reusing the alphabets previously inputted. The Hangul modification system using "Jamo Modification Window" follows the composition principle of Hangul, and utilize the already inputted alphabets. It can be applicable to the existing input system without any modification using only "Jamo Modification Window". Especially, this system is very useful on smart phone with small screen.