Introduction: This study was performed to investigate the frequency of iron-deficiency anemia and some related factors in children with cyanotic congenital heart disease between 2011 and 2019 in Mashhad City, Iran. Method: This cross-sectional observational study was conducted with a consensus method on children under 14 years of age. The samples were diagnosed with cyanotic congenital heart disease in the pediatric heart department of Imam Reza Hospital in Mashhad and had an oxygen saturation of less than 94%. The information from the patients' files was recorded in the questionnaire. The studied children were divided into five age groups to evaluate iron-deficiency anemia. The mean and lower limit of normal erythrocyte indices were determined, and the patients with anemia were identified based on mean corpuscular hemoglobin (MCH), mean corpuscular volume (MCV), and red blood cell distribution width (RDW). The data were statistically analyzed in SPSS23 software using Chi-square and Mann-Whitney statistical tests. A significance level of 0.05 was considered. Results: Among 250 examined patients, 90 cases were included in the study (54 boys and 36 girls). Angiography was the reason for half of these patients' referral to the hospital. The highest frequency was related to patients with tetralogy of Fallot (40%). The frequency rates of iron-deficiency anemia based on MCV and MCH erythrocyte indices were 10% and 17.8%, respectively. Moreover, 64.4% of patients had increased RDW levels. Cyanosis attacks were reported positive in 85.4% of the studied population. Of note, the frequency of cyanosis attacks was not significantly different in the two gender groups (P<0.494). Conclusion: Considering the high prevalence of iron-deficiency anemia, especially in children with congenital heart disease, the treatment of this disorder is necessary for all such children and should not be neglected