Background: Low birth weight has been defined by the World Health Organization (WHO) as weight at birth of less than 2500 grams. Any baby born prematurely is more likely to be small. However there are other factors that can also contribute to the risk of low birth weight, these include: race, mothers’ age, multiple births, mothers’ health and mothers of lower socioeconomic status. The incidence of low birth weight is monitored through both health system surveillance and household surveys. Among regions, South Asia has the highest incidence of low birth weight, with one in four newborns weighing less than 2500 grams. In a study in Iran the prevalence of LBW was estimated 7 percent (SD 95%), prevalence of LBW is different according to the geographical region. Methods: Demographic data got from mothers and children height and weight measured by trained staffs in and out of hospital and health facilities. Results: Total number of births in Iran in 2015 is 1,509,081 that from this 777,351 are male, 731,014 are female and 716 ambiguous. The percentage of LBW in male, female and ambiguous is 6.5, 7.7 and 53.9 respectively. The highest percent of LBW in female was related to Sistan Blochestan province with 12.6 and the lowest percent to Lorestan province with 5.6 percent. Discussion: According to the guidelines published by World Health Organization (WHO), any neonate with a birth weight less than 2500 grams is considered to be LBW. LBW and PTM are leading causes of adverse perinatal outcomes and are closely related to neonatal diseases and deaths. In the world, there are more than 20 million LBW infants born each year that 95.6% are born in developing countries. In the present study, the number of LBW neonate in male, female and ambiguous was 50,569, 55,972 and 386; the percentage of LBW in male, female and ambiguous was 6.5, 7.7 and 53.9 respectively. Regarding the prognosis of LBW, with development of perinatology, the survival rate of LBW infants has substantially increased. However, some adverse outcomes are still relatively common, especially long-term complications such as cerebral palsy, delayed neurodevelopment and visual and hearing impairments. Conclusions: The incidence of LBW in Iran is higher in deprived provinces such as Sistan Blochestan in southeast of Iran.