A method is developed to quantify the particle concentration non-uniformity of cohesive powders in a simple shear flow for both dilute and dense conditions. The ratio of the variance of the particle number concentration, rt, to the square of mean particle number concentration, r = [n'2]/[n](2), is first obtained using a series of sub-cells of different volumes based on a discrete element simulation of particle dynamics in a simple shear flow. The deviation, ar, of r from the rapid shear rate limit at the same bulk concentration is taken as the measure of the particle concentration non-uniformity. The effects of shear rates on this concentration non-uniformity are quantitatively examined. The quantitative value of this non-uniformity is used to identify a change in the micro-scale cluster structure of dense powder flows under various shear rates and to help in understanding the mechanism for the observed non-monotonic stress-strain rate behavior.