Management of open apex was traditionally by Calcium hydroxide [Ca(OH)2]. Recently, alternatives like biodentine has gained importance. Aim: This study was done to compare the sealing ability of 2mm Vs 5mm of biodentine as apical plug. Materials and methods: 30 mandibular premolars were decoronated and the apices were removed up to 2mm. Canals were instrumented and the specimens were divided into 3 groups – Group A– Control–(Untreated), Group B–2mm and Group C–5mm biodentine apical plug respectively. Group B and C were then obturated and the coronal access were sealed with composite resin. The samples were then immersed in 1% methylene blue for 72 hours. Then, samples were split longitudinally and prepared for stereomicroscopic evaluation. Statistical analysis of data was done using One–way ANOVA. Results: There was a significant difference in the sealing ability of 5mm when compared to 2mm of Biodentine. Conclusion: Greater thickness of Biodentin has better sealing ability.