The ritual and social organization of a tribe reflect her religion, social life and norm values. The main traditional ritual in Atayal society is agricultural ritual. Social organization Ga-Ga is an important part of her traditional culture. Atayal people live under the constraint of Ga-Ga group in the past. "Rutux" is her basic belief and "Ga-Ga" is her ritual unit, which is based on blood relationship and obeys the same habit, law and taboo. Ga-Ga has at least 5 meanings:1. blood group with the same habit, 2. a group to reinforce Ga-Ga's social functions through holding rituals, 3. a group where every member possesses economic interests, 4. a geographical group which is formed in the process of migration, 5. a group constrainted by habits and moral standards which are inherited by the oral way. Because Ga-Ga plays the role of law and moral norms, traditionally Atayal society had good social norms. The main method of this study is histrical document analysis, and it is supplementary by anthropology's field investigation. It also analyses Atayal's Ga-Ga and Rutux through the view of social function. The goal of this thesis is to understand Atayal's social organization and norms through inquiring into Ga-Ga. First, it synopsises Atayal tribe.secondly, it explores the tribes organization through traditional agriculture ritual, and inquires the meaning and the functions of social norm. Then it explains the disintegration and transformation of Ga-Ga's functions. The author also went to Atayal tribe to interview with Atayal people. Finally, it suggests that the functions of Ga-Ga should be reconstructed through school education.