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Background: The purpose of this study was to identify the effect of health education according to health belief model (HBM) on the adoption of preventive behaviors of infection with giardia lamblia by primary school students in Arak. Materials and Methods: In a randomized quasi-experimental case study, 1200 students were randomly divided into two equal groups of test (instruction) and control (non-instruction). The sample size for giardiosis identification tests included 300 students, who had been equally and randomly selected from the control and test groups. Health education was according to HBM and its impact on the students' lifestyle-related patterns of behavior was considered. Results: Paring nails in the test group, significantly increased after education while it decreased significantly in the control group. The number of students who carried a glass for drinking slightly increased, but it remarkably decreased in the control group. The number of students who applied soap after using the restrooms remarkably increased in both the test and control groups. The relationship between not paring nails and infection with giardia lamblia was statistically significant. Additionally, after education, the ratio of reduction of infection with giardia lamblia in the test group to its increase in the control group was statistically significant. Conclusion: Preventive behaviors significantly improved and appeared influential in the reduction of infection with giardia lamblia. Thus, using a HBM in prevention of parasitic infections along with instructing the parents are suggested for prevention of parasitic infections.