Weight-gaining refers to providing sufficient calorie, liquids, and nutrients and appropriate growth for the newborn baby, so training the appropriate feeding methods improves weight-gaining. The present research is aimed to investigate the effects of counseling the mothers on weight-gaining of the preterm infants hospitalized in Fatemieh Hospital in Hamadan. The randomized clinical trials on 124 individuals from among the mothers with preterm babies hospitalized in Fatemieh Hospital in Hamadan were investigated and the samples were randomly categorized in two control and experimental groups. A researcher-made checklist was used for examining the breastfeeding performance and five sessions of breastfeeding consultation were held for the intervention group. In order to examine the babies’ weight-gaining, a baby-weighting scale of co12000 model, made in Iran, was used. Finally, the information and the significance level P<0/05 were analyzed using the Software SPSS Version-21. Comparison of the preterm babies’ weight-gaining in control and experimental groups showed no significant difference before intervention (P=0/86), but this difference was significant during the first to fourth month of followup( P<0/001). so that the average weight of the preterm babies in both control and experimental groups in all four months of follow-up had increased compared to the period before the intervention. Findings showed that consultation improves the continuation of exclusive breastfeeding, increases the number of breastfeeding sessions, reduces the use of liquids and dry milk instead of breastfeeding for the preterm babies, and increases the babies’ weight-gaining.