The present study is aimed to explore the potential to improve Vertical Axis Tidal Turbine (VATT) energy harnessing efficiency by using modified blades with fixed and oscillating flap. The fixed flap concept is borrowed from its application in aerodynamics area for reaching a high lift force at low flying speed. Oscillating flap is motivated by our relevant biomimetic studies on the flapping wing propulsion or energy extraction. Present investigation is concentrated on a VATT with NACA 0018 profile blade as its baseline turbine. Numerical simulations are carried out by solving incompressible Unsteady Navier-Stokes equations at turbulence flow condition. Computed results show that under certain optimal flap geometry and flow conditions, turbine power coefficient reaches 28% enhancement as compared to the conventional blade turbine. Detailed analysis on the flow structure demonstrates that this is related to the effective flow separation suppression and vortex control by applying a fixed and oscillating flap. (C) 2012 Elsevier Ltd. All rights reserved.