Introduction: A large proportion of under-five-year kids are suffering from malnutrition. A study was conducted to assess the knowledge of mothers regarding malnutrition and its prevention and evaluate effectiveness of structured teaching program regarding malnutrition and its prevention in terms of knowledge gain in mothers of under-five children attending a crèche run by an NGO in a slum area of New Delhi.Methodology: Quantitative research approach with one group pre-test, post-test design was used. Tool used for generating necessary data was a structured knowledge questionnaire, after establishing its validity and reliability. Purposive sampling technique was used to select 45 mothers having children under five years of age. The study was conducted at a crèche run by an NGO in a slum area of Delhi.Results: Before administration of the structured teaching program, 18 (40%) mothers had poor knowledge, 15 (33.3%) had average knowledge and 12 (26.7%) had good knowledge about malnutrition and its prevention, while after administration of structured teaching program, 12 (26.7%) had poor knowledge, 21 (46.7%) had average knowledge and 12 (26.7%) had good knowledge about malnutrition and its prevention indicating that the intervention was effective. There was significant relationship between knowledge gain and age, education and monthly family income of mothers. Conclusion: Finding of the study revealed that mothers having children under five years of age had poor knowledge about malnutrition and its prevention. The structured teaching program was an effective tool to enhance the knowledge of mothers