Mixing of particles in a rotating drum mixer with filling level greater than 50% has been analysed using Discrete Element Method (DEM). An attempt has been made to understand the mechanism of the dead zone formation and the degree of mixing by varying the mixing parameters. These include the size and packing of particles, speed and shape of the mixer, etc. While the formation of the dead zone is qualitatively analysed, the degree of mixing has been quantified with a suitable mixing index (psi). It is found that packing arrangement and particle size significantly affect the formation of the dead zone, whereas, the drum speed and geometry has a relatively lesser effect. The effect of various parameters on the dead zone formation is explained on the basis of variation in energy distribution pattern from the wall towards the centre of the mixer. It has been found that the energy required for agitating the particles is transferred from the outer wall to the centre of the mixer. In the process of transfer, a significant amount of energy is expended before reaching the centre, which allows a compacted dead zone to form around the centre. A preliminary attempt has been also made to study mixing in a drum with baffles. The baffles act as the medium of efficient energy transfer by imparting their energy to the particles that come in contact with them thereby resulting in better mixing. (C) 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.