Aim: The effect of Significant Caries Index application on the caries risk of undergoing orthodontic treatment patients. Material and Method: Ninety subjects who were undergoing fixed orthodontic treatment were evaluated for this study. All patients were treated with a 0,018 inch slot preadjusted fixed orthodontic appliances. An examiner used the Gorelick index for assessment of white spot lesion on the buccal surface of teeth before (T1) and after (T2) treatment. The WHO (World Health Organization) criteria and bitewing radiographs were utilized to diagnose the carious status of the subjects. Subjects were divided into three groups (G1,G2,G3) according to Significant Caries Index. Results: 72 patients completed study (38 female, 34 male) whose mean age 14.7±1.63 at T1. The mean DMFT index was 3.18 and the SiC was 5.00. The mean DMFT increase was found 0.14 for all subjects. The prevalence of white spot lesion is 16.6% at T1, 59.6% of patients presented white spot lesion at T2. White spot lesion incidence was found 40%. No significant differences was found between risk groups which is formed according to Significant Caries Index, in terms of DMFT increase, occurrence and severity of white spot lesion (p>0,05). Conclusion: It was found that Significant Caries Index application are not play a decisive role on the formation of white spot lesion and DMFT index change in patients with undergoing orthodontic treatment