Introduction: Acute appendicitis is one of the main cause of acute abdomen referral to emergency departments worldwide. And despite recent investigations in diagnostic methods, appendicitis accurate diagnosis is still one of the controversial issues in surgery. It is estimated that incidence of negative appendectomy in men and women varies from 8% to 45%. In this study we compared the accuracy of ultrasound and Computer tomography scan (CT scan) in patients suspected to acute appendicitis.Material and Methods: In this cross-sectional study, patients were enrolled the study who were referred to emergency department of Imam Reza hospital with clinical suspicion ofacute appendicitis. Radiologic result of patients underwent surgery for acute appendicitis was compared with pathology reports. Data were analyzed by chi-square and T-student tests in SPSS16.Results: Overall 46 males and 35 females (mean age: 24.6±8.9 years) were studied. Sensitivity, specificity, positive predictive value and negative predictive value of abdominal ultrasound for acute appendicitis were 73.9%, 62.5%, 94.4%, and 78.2%, respectively. These values for CT scan were 88.8%, 80.9%, 94.1% and 66.6%, respectively.Conclusion: our results showed that CT scan is more accurate than sonography for diagnosis of acute appendicitis, particularly in female patients and patient with nonspecific symptoms.