The objective of this research was to examine the factors affecting birth control behavior of the Hmong ethnic group. The multi-stage random sampling method was utilized to select 341 Hmong women who implemented a birth control method and resided in Chiang Mai, Chiang Rai, Mae Hong Son, Nan and Tak provinces. Interviews were used to collect the data and the quantitative data were analyzed and tested with Chi-square. General and birth control data were summarized, such as, ages of the sample group and of their husbands, number of family members, occupations of the sample group, number of pregnancies and deliveries, participation in limiting the number of children, reasons of birth control, and places to receive birth control services. When the factors were related to birth control methods, it was found that the age of the wife, the age of the husband, the religion, education, number of family members, number of live children, number of male children, number of deliveries, housewife, knowledge sources, service provision sources, post-natal care services, travel expenditure, and service expenditure were related to the birth control practices of the sample group.