Based on previous research studies the effects of operational conditions as mill loading and rotation speed on flow properties such as flow velocity, powder draw, collision frequency and total impact energy were investigated in a horizontal high speed stirred mill with a similar configuration to that of an Isa MillTM. Numerical simulations based on discrete element method were performed to investigate the particle flow. The results show that increasing mill loading or mill speed increases collision energy between particles. Moreover physical experiments were carried out to quantify grinding performance under similar conditions. The results show that dry grinding in a stirred mill follows a first order kinetic process with particle size decaying exponentially with time. The rate constant can be correlated with the total impact energy. The results indicate that the total impact energy, obtained from simulation, can be useful index to predict grinding performance at the process equipment scale. The proposed methodology may also apply to other stirred mill.