Introduction: Appendicitis is a common urgent surgical operation. Since early diagnosis and treatment of appendicitis is necessary and also some disease mimic its clinical manifestation and rate of negative appendectomy was high (15-30%), so surgeons were trying to increase the accuracy of diagnosis of appendicitis by using laboratory facilities. The main aim of this study were to investigation of Sensitivity and Specificity CRP in the diagnosis of acute appendicitis and compare it with routine laboratory tests such as leukocyte count and neutrophil count.Material and Methods: In this descriptive - analytical study, blood samples of 191 patients with diagnosis of acute appendicitis that underwent appendectomy at the Besat hospital, were used to measuring of CRP. The surgical specimens were reviewed by pathologists and pathologic findings were considered as gold standard. Then by using of the results of pathology tests, sensitivity, specificity and positive and negative predictive value of CRP, WBC and neutrophils were measured. Data were analysed by SPSS-14 software and t test and chi square test.Results: 74/3% of patients were male and 25/7% female and the mean age was 26 ± 3/4 years. The most common symptom, sign and pathologic stage of patients were loss of appetite (95/2%), local tenderness (95/8%) and Acute suppurative (64/9%). 08/13% of patients had normal appendectomies that consist of 35% of women and 6% of men appendectomy. The sensitivity and specificity of CRP, WBC and neutrophils respectively were: 98/1%, 61/5%, 84/8%, 50%, 93/3% and 19.2%.Conclusion: According to the results, the CRP test at diagnosis and in finding of negative appendectomy has high value. But wasn't reliable as an exclusive test. Therefore, the use of CRP as a paraclinical criterion along with other criteria of Alvarado scoring system can be very useful in diagnosis of appendicitis.