For the purpose of choosing and designing measurement algorithms to characterize particle size and shape rotation-invariantly, reproducibly, simply, and reasonably, this paper presents an image analysis measurement algorithm-best-fit rectangle for particle size and shape. The best-fit rectangle approach is a combination of the Ferret method and the least 2nd moments minimization, only requiring calculation of three moments about the center of gravity, and maximum and minimum co-ordinates in a co-ordinate system oriented in the direction of the axis of the least 2nd moments, and a simple area ratio. It is a simple rotation-invariance method, reflecting shape (elongation and angularity). In this paper, the method that has been tested in a large number of aggregate particle samples in a laboratory is introduced theoretically in detail, analyzed and compared to other widely used methods. The test results show that by using this method, the results are very close to manual measurements (for size, elongation and angularity). The width accumulative curve is parallel to the curves of sieving and thickness, implying that sieving analysis and thickness measurement can be easily estimated by the width. The method combining other image processing algorithms in an online system does the processing in real time. It is more reasonable and useful than other traditionally used measurement methods in image analysis. (c) 2006 Elsevier B.V. All rights reserved.