In order to investigate the classification performance of static air classifier in an industrial Vertical Spindle Mill (VSM), sampling ports were drilled on the mill shell and new on-line sampling methods were designed. Size and density compositions of classifier feed, classifier reject and Pulverized Fuel (PF) were analyzed. Calculation was done about the circulating ratio within the VSM and classification efficiency of the classifier with several surveys. Influences of three operation variables, namely coal type, air rate and roller load pressure on the classifier performance were investigated. Result shows that circulating ratio is around 7 and 10. Yield of particles with -90 mu m decreases from 32% in classifier feed to 24% in classifier reject after classification, which indicates the poor classification of classifier. Density composition of samples in classifier feed, classifier reject and pulverized fuel are quite different. Dominant density levels are -1.4 g/cm(3) and +1.8 g/cm(3) in pulverized fuel due to size reduction and liberation. Results of classification efficiency show that the variation of the air rate in a range between 62.8 t/h and 53.2 t/h has limited influence on the efficiency, however, classification efficiency decreases significantly when air rate is smaller than 44 t/h. In addition, both the coal type and the roller load pressure have indirectly influence on the classification efficiency by affecting the flow rate of the classifier feed. (C) 2016 Elsevier Ltd. All rights reserved.