In this work, the effects of trailing edge (TE) cutback on the turbulent flow field and the aerodynamic loss of a turbine blade are investigated by Large-Eddy-Simulation (LES). A high-fidelity LES code was first developed based on a high-order finite difference method and applied to the simulation. The effects of the TE cutback on the blade surface boundary layer, pressure distribution, and loss generation are then discussed on the basis of time-mean flow fields of LES. The results represent that the cutback changed the boundary layer thickness just upstream of the TE and the base pressure around the TE, resulting in larger aerodynamic loss. Next, the time-dependent flow dynamics in the cascade is investigated. The results indicate that the dominant pressure fluctuation after the blade was influenced by the cutback. Furthermore, the phase-locked flow field is analyzed to reveal the flow dynamics responsible for the pressure fluctuation. Finally, the LES approach could clarify the underlying mechanisms of performance degradation of a turbine blade that caused by TE cutback.