This study was designed to evaluate diagnostic value of C-reactive protein in determining of gestational diabetes mellitus. The present case-control study was conducted on 60 pregnant women with GDM (case group) and 120 women with normal pregnancy (control group) referred to Ayatollah Taleghani and mahdiyeh Hospitals. The serum level of qualitative and quantitative CRP was measured and diagnostic value of CRP was determined. In this study information form was completed by interview and sampling was performed by convenience method. Data were analyzed by SPSS-17 and significance level of p<0.05 was considered. Serum CRP value in GDM and control groups was 3/ 59±3/2 and 1/44±3/3 mg/liter, respectively. Regarding to cut-off of 2/2 mg/liter, sensitivity, specificity, positive predictive value and negative predictive value of quantitative CRP were 71%, 60%, 47% and 81%, respectively in diagnosis of GDM. The under curve area was 0/70. Sensitivity, specificity, positive predictive value and negative predictive value of qualitative CRP in diagnosis of GDM was calculated 33/3%, 98/3%, 90% and 74%, respectively. It seems that measuring CRP in pregnant women with GDM risk factors can be used as a simple, new and reliable method to screen gestational diabetes mellitus.