Nutrition is a key factor in nutritional development. Nutritional wellbeing is a sustainable force for health and development and for maximization of human genetic potential. Lactation is a physiological process, which has profound relevance for both the mother and the new born. Adequate nutrition for the mother during lactation is also of vital importance as the infant is dependent on mother’s milk for its nutrition for the first few months of life. As the mother has to nourish the fully developed and rapidly growing infant mothers need extra nutrients to meet the baby’s increasing needs. A quantitative study was chosen to assess the effectiveness of self-instructional module on knowledge regarding diet during lactation among Primipara mothers in a selected hospital. 100 samples were selected who come under the inclusion criteria by simple random technique. Data was collected using demographic variables which include Age, type of family, religion, occupation, education, frequency of breast feeding, duration of breastfeeding. In pretest the values of poor mean (2.66) and standard deviation (1.633), moderate mean (7.93) and standard deviation (1.389) and adequate mean (12.909) and standard deviation (1.445). In posttest the values of poor mean (3.25) and standard deviation (0.957) moderate mean (8.031) and standard deviation (1.331) and adequate mean (12.968) and standard deviation (1.368) and paired t value is 11.452 and posttest is statistically significant. The study indicates that after giving self-instructional module the knowledge regarding diet during lactation among the Primipara mothers is effective.