PURPOSE: To evaluate the accuracy of X-ray evaluation in classification, displacement and size of posterior malleolar fragment, comparing with three dimensional computed tomography (3D CT) in trimallelar ankle fractures. MATERIALS AND METHODS: 20 cases of trimalleolar ankle fractures evaluated with preoperative 3D CT, and followed up periods were at least 2 years. All cases were classified according to the Danis-Weber and Lauge-Hansen classification. Displacement and size of posterior malleolar fragment were measured using PACS. The reliability between simple X-ray and 3D CT was evaluated in the Danis-Weber and Lauge-Hansen classification (kappa analysis). The correlation between simple X-ray and 3D CT was evaluated in displacement and size of posterior malleolar fragment (correlation analysis). RESULTS: Degree of agreement of Danis-Weber classification in simple X-ray and 3D CT was 0.700 kappa value, and that of Lauge-Hansen was 0.605 kappa value. Measurement of simple X-ray and 3D CT about displaced status of posterior malleolar fragment showed statistically significant positive linear correlation (p= 0.000), but correlation of measurement of size in simple X-ray and CT was not statistically significant (p=0.102). CONCLUSION: CT or operative field will be more accurate than simple X-ray to select the method of treatment and operation, especially when the displacement and size of posterior malleolar fragment are important to decide.