Background and purpose: Dental decay is the most prevalent infectious disease. One of the dentists’ major disturbances is accurate diagnosis of small proximal decay in posterior teeth. Different diagnostic methods have been offered to detect decay since before. One of them is radiography that consists of different methods and machines. The purpose of this study was to evaluate the diagnostic accuracy of intra-oral digital radiography in detection of interproximal decay of posterior teeth. Materials and methods: This cross-sectional In vitro study was conducted with the 140 interproximal surfaces of patients’ molar as sample size. In half of them, decay was simulated by drilling and were mounted on the cast and then digital bite wing radiography was taken using PSP diagora optime sensor. Results: Diagnostic specifity was 71% and 69% for mesial and distal surfaces respectively. No significant relationships between detecting interproximal decays in mesial and distal surfaces of teeth were observed after statistical analysis using digital BW radiography. Conclusion: Results of this study concluded that there are no meaningful statistical differences in diagnosis of interproximal decays between mesial and distal surfaces of teeth while using BW radiography. Sensitivity and specialty of BW technique in diagnosing interproximal decays is lower than diagnosing decays larger than 0.5mm, however considering the positive and negative predictive value, this technique can still be used to for periodic examinations and patients` follow-ups in society due to its high sensitivity.