Postpartum period is an important step because the mother's body needs to heal and recover, after pregnancy period and childbirth. So taking good care and a balanced diet in the postpartum period, plays an important role in the women's health. Thus, training of nutrition is so necessary at this period. The aim of this study was an evaluation the effect of the pregnancy period preparation classes on the nutritional behavior of the postpartum period. This research is a semi-experimental study. 230 numbers of the nulliparas admitted to the health centers selected by stratified-cluster sampling method and were placed in the intervention and control groups. Classes for the intervention group were held once every one week in the eight sessions and the duration of each session, was one hour. Collection of data was conducted through questionnaires and in order to its analysis, SPSS software was used. The results for this experiment showed that, the groups of intervention and control groups, were similar in terms of the demographic characteristics but there was a significant difference in nutritional behavior of these groups and the amount of food groups and duration of iron tablet's consumption for the intervention group, were more than the control group (P=0.000). Therefore, the final results from this investigation showed that, classes of the pregnancy period preparation had positive effects on the nutritional behavior and consumption amounts for the food groups in the postpartum period which indicates the importance of pregnancy period classes and role of midwives in the field of the pregnancy period training.