Mill chatter is a notorio/ls condition in the high speed strip rolling process, having a behavior that is complex and changeful. During fieldwork to measure the vibration in high speed rolling mills, a frequency modulation phenomenon was identified, which had not beea reported in the strip rolling community. This article makes a theoretical investigation of the newly encountered phenomenon in the strip rolling process. According to an analysis of the mill roller bearings, the periodic change of stiffness of the bearings at the fixed load orientation, together with the periodic change o f the effective en-ors of the bearings, may ca use mnplimde mul frequency modulation, based on the theories of Fourier series and Bessel functions. The compmison between test results and theoretical analysis implies that the frequency modulation is caused by the effective en-ors of the mill roller bemings, that have 12 ripples on the internal ring.