Statement of Problem: The treatment of immature teeth with open apex by Gutta percha in one visit apexification requires an optimum material as apical barrier. One of the desirable properties of the apical barrier material is its sealing ability.Purpose: This study was performed to compare the sealing ability of calcium hydroxide and a new experimental material (cold ceramic) when used as an apical barrier.Materials and Method: Fifty extracted single canal human teeth were collected and stored in saline solution. The crowns were resected and by conventional step back method, canals were instrumented and then extended by Piso Reamer, so that, root ends were formed divergently. Samples were divided into two group including 23 teeth as study and 4 teeth as positive and negative control groups and then all teeth were mounted in a plaster cast. The two experimental materials (Cold ceramic and calcium hydroxide) were placed in the root ends for producing apical barrier, and canals were obturated with Gutta percha. The teeth in the positive and negative groups were obturated with Gutta percha, and in the negative group, cavit was placed at the end of root. In the 3 groups (calcium hydroxide, cold ceramic and positive group), all surfaces of the teeth were covered by two layers of the nail polish except 2 to 3 mm of the roots end and in the negative control group, samples were covered completely by two layers of the nail polish. Then, photographs of teeth were scanned and amount of leakage was calculated with the Photoshop software. The data were analyzed by t-test.Results: Amount of cold ceramic leakage was less than calcium hydroxide and this difference was statistically significant (p=0.0001).Conclusion: Results showed that apical plug with cold ceramic was more resistant than the apical plug with calcium hydroxide. The results of this study confirmed optimum sealing ability of the new experimental material for creating of apical barrier in one appointment apexification. Key words: Apexification, Calcium hydroxide, Cold ceramic