Background The present study aimed to explore effects of companionship in training sessions on primiparous women's knowledge, attitude and performance toward breastfeeding. Materials and Methods: In this case- control interventional study, 105 primiparous women in the 30th to 34th weeks of pregnancy were selected and assigned to three groups, namely the intervention group without a companion (n=35), the intervention group with a companion (n=35), and the comparison group (n=35). Data gathering tools was a questionnaire and breastfeeding observational checklist. The control group received routine training in the hospital, but the other groups were trained by the researcher in four 90-minute sessions. Data were collected before and after the education, immediately after childbirth, and four and eight weeks after childbirth and analyzed using SPSS(version 20.0) software and appropriate statistical tests. Results: No significant difference was observed in terms of demographic variables such as: maternal and paternal educational level and occupation status between control and intervention groups (P > 0.05). There was a significant difference between the mean scores of knowledge and attitude toward breastfeeding in the two intervention groups before and after the educational intervention, right after child birth, 4 weeks and 8 weeks after child birth (P < 0.001). There was also a significant difference between the three groups in the mean score of performance of breastfeeding right after, 4, and 8 weeks after childbirth (P < 0.001). Conclusion: According to results of this study, education during pregnancy and support from the accompanying person, before and after childbirth expand knowledge of breastfeeding, reinforce attitude toward it, and boost its performance