The thesis examined factors affecting birth control practices of Yao women in four villages in Ngao District, Lampang Province. It was found that the factors included the number and gender of children, distance from home to a public health center, birth control experience, e.g. side effects, fear from side effects, and rumors about the side effects of birth control. Other factors not affecting the birth control practices were creeds, family types, knowledge and perception about birth control, service centers, information sources, attitude toward, traditions and customs about birth control, and treatments during sick periods. Generally, the Yao women tended to accept birth control plans, however, birth control was hardly practiced in some villages due to insufficient knowledge and understanding about the practices, a social value of preference to have sons, the distance of the village from a public health center, and a higher reproductive rate. It is recommended that knowledge about birth control be rigorously disseminated, which are the promotion of accepted birth control methods: injection and sterilization, and expansion of birth control services to cover wider target groups.