BACKGROUND Iron deficiency anaemia is one of the most common nutritional problems in Iran, especially in women and adolescent girls. Iron supplementation is the most cost-effective and most common strategy used by developed countries to control the iron deficiency. The purpose of this study was to investigate the effect of education based on the theory of planned behaviour on consumption of iron supplement in secondary school girls in Kermanshah, 2017. MATERIALS AND METHODS This quasi-experimental study was carried out with 174 students, girls of first grade of secondary high school from Kermanshah city. The subjects were selected by multistage random sampling method. The students were divided into three groups including student (n= 58), student with mother (n= 58) and one control group (n= 58). A researcher-made questionnaire based on the theory of planned behaviour was used to collect the data. Participants completed self-report questionnaire on demographic variables and theoretical constructs. Then based on the initial results, the educational content was developed. The data were collected immediately and one month after the implementation of the training program it was analysed with SPSS 20 software using Kruskal-Wallis, Mann-Whitney and Friedman tests. RESULTS The mean scores of knowledge, attitude, perceived behavioural control, subjective norms, intention and behaviour of iron supplementation immediately and one month after intervention was significantly increased in intervention groups (P < 0.05). However, there was no significant difference between the two intervention groups, although there was a better situation in the intervention group with the mother. CONCLUSION The results of the research show that educational intervention based on the theory of planned behaviour can affect all the structures of the theory and ultimately change the behaviour of the consumption of iron tablets in students.