This study was carried out to assess knowledge regarding breast feeding and weaning among lactating mothers of infants attending International Centre for Diarrhoeal Disease Research, Bangladesh (icddr,b) Hospital. A pretested semistructured questionnaire was used for this purpose and the study period was May to August 2011. 200 lactating mothers with children less than one year of age was selected. The study revealed that the mean age of mothers was 24± SD 6 years and mean age of child was 6.25 months. Most of respondents were Muslim (92.5%) and from nuclear family (62.5%). Most of the respondents had completed their primary education (56%), 82% of them are house wife, most of the respondents family income ranges from BDT 3,000-10,000. Majority of the respondent had mean monthly income of BDT 6028 ± SD 4636. Marital age’s ranges from 18-22 years and 23% were less than 18 years. Majority of infants’ mother (81%) thought that breast feeding is compulsory for every child. About (46%) opined that colostrum is needed for infants. About 26% thought that thickness of colostrum is indigestible and 22% thought that colostrum may be harmful for the infants. Only 33.5% had fair knowledge on breastfeeding but majority (72.5%) of the infant’s mother shows fair knowledge regarding weaning. About 75 % mother got information regarding breast feeding and weaning knowledge from television. The knowledge of mothers regarding breast feeding was poor but their knowledge on weaning was comparatively adequate.