This paper presents an analytical model to monitor the gradual wear of cutting tools, on-line, during turning operations using ultrasound waves. Ultrasound waves at a frequency of 10 MHz were pulsed continuously inside several cutting tools, towards their cutting edge. The change in tool geometry, due to gradual wear, has been related, in a mathematical form, to the change in the acoustic behavior of ultrasound waves inside the body of the cutting tools. Physical laws governing the propagation and reflection of ultrasound waves along with geometrical analysis of the wear area were used in deriving the mathematical model, The experimental setup and model evaluation is based on a previously published research work by the author, which presented an empirical model showing a corresponding change in the ultrasound behavior with tool gradual wear. The current work emphasizes the previous findings and presents the relation between the acoustic behavior of ultrasound waves and the progressive tool gradual wear in a mathematical form that can be easily used in machine control operations. (C) 2000 Elsevier Science Ltd. All rights reserved.