This paper reports numerical investigations performed to optimize the mixing performance of a radial grid disc impeller in a stirred vessel. In the author's previous work (Buwa et al., 2006, Chem Eng Sci, 61: 2815-2822), a novel grid disc impeller having radial flow characteristics was developed, numerical simulations of the flow generated by the grid disc impeller were performed and the predictions were validated experimentally. In the present work, the numerical simulations were performed to investigate the effects of the rotational speed, grid opening size and impeller diameter on the mean flow generated by the various impeller configurations, mixing performance and power requirements. Numerical simulations were also carried out to evaluate the performance of a dual radial grid disc impeller. The results show that the mixing efficiency per unit power consumed improves with an increase in the grid opening size and also with an increase in the diameter of the grid disc impeller. Further, the mixing efficiency was significantly improved for the dual grid disc impeller with large grid openings.