To achieve improvements in the production capacity and energy efficiency of an industrial tubular ball mill, an Improved adaptive Evidence-Theoretic k-NN rule was proposed, and was applied to monitor an unmeasured parameter, i.e., level of coal powder filling in ball mill. The improved adaptive rule was realized by means of two strategies: (1) a parametric distance metric was applied instead of Euclidean distance metric, and (2) some structure parameters fixed in the original adaptive rule, as well as the parameters brought in by the parametric distance metric, were optimized based on gradient descent algorithm. Some popular data sets were used to validate the performance of the improved adaptive rule. Consequently, the improved adaptive rule was applied to monitor the level of coal powder filling in ball mill. To demonstrate the validation of the improved adaptive rule for this task, some experiments were conducted on an industrial tubular ball mill. The experimental results suggest that the improved adaptive rule was applicable for monitoring the unmeasured parameter in industry. (C) 2009 Elsevier Ltd. All rights reserved.