This work investigates plasma performance of DEMO plasmas in the presence of internal transport barrier. The study is conducted based on the simulation results carried out using a BALDUR integrated predictive modelling code. In these simulations, a combination of a neoclassical transport model NCLASS and an anomalous transport model Mixed B/gB is used. The boundary condition is described at the top of the pedestal, which is calculated theoretically based on a combination of magnetic and flow shear stabilization pedestal width scaling and an infinite-n ballooning pressure gradient model. The toroidal flow calculation is based on NTV (neoclassical toroidal viscosity) toroidal velocity model. Time evolution of plasma temperature and density profiles of ITER- and DEMO-like (European PPCS and Chinese HCSB-DEMO) plasmas are simulated with internal transport barrier (ITB). Densities of impurity (Beryllium and Carbon) and particle are compared among all scenarios. The aim of this study is to identify the optimization point between plasma performance and impurity accumulation. Though transport barriers can improve plasma performance, the significant accumulation of impurity in plasma core can lead to an enhancement of radiation loss.