Introduction: The period of breastfeeding in all societies is one of the most important health factors. The pattern of breastfeeding is different in most societies, so it is important to know this pattern in every population. This study was performed to investigate the period of breastfeeding in under 36 month old children at Zahedan. Material and Methods: This study was performed in late 2004 and early 2005 at Zahedan. The sample size was 450 mothers with at most 36 months old children which had visited the 5 health centers of Zahedan in 2004. The sample size in each center was equal and it was chosen using cluster random sampling. Every mother were asked about her and the child age, the period of breastfeeding, the education and job of the mothers, number of children, the age of complementary feeding, kind of delivery, number of breastfeeding in 24 hours, the situation of mother, breastfeeding at nights, breastfeeding to ill child and breastfeeding of ill mothers to their child. Results: The median of breastfeeding was 15 1.17 month. The 98% of the babies start breastfeeding after the birth. The proportion of breastfeeding in months 1, 3, 6, 12 and 24 were 92%, 85%, 69% and 8% respectively. The period of breastfeeding has a significant relation to the age of the baby and mothers, mother education, breastfeeding at night, breastfeeding to ill child, breastfeeding of ill mothers to their children and the number of breastfeeding at 24 hours. There is no significant relation between the period of breastfeeding and the job of mothers, the number of children, the mother situation, the kind of deliveries and the age of complementary feeding. Also, the period of breastfeeding reduces during the last three years. Conclusion: To avoid the decreases of the period of breastfeeding, it is essential to have some health education to young and pregnant mothers. Also, it is necessary for the health authorities to plan some program to prevent this problem.