This paper iticludes the findings of an experitnental study on instabilities of the chip formation process during end milling ofTi6Al4V alloy and the infiuence of these instabilities on chatter formation. It has been identified that the chip formation process has a discrete nature, associated with the periodic shearing process during machining. The chip formed during machining of titanium alloy TÍ6A14V is found to be mainly with primary serrated teeth appearing in the main body of the chip. Secondary .serrated teeth resulting from the coagulation of a certctin number of primary serrated teeth also happen to appear at the free or constrained edge of the chip, especially when the .nstetn enters into chatter. In order to identify the interaction of these chip instabilities with the prominent natural vibration of the machine tools system components, the different mode frequencies of the vibrating components of the system have been identified using experimental atid finite eletnent modal atialyses, and vibration responses during actual cutting have also been recorded using an online vibration tnonitoring system. The vibration signals in frequency domain (fast Fourier transform) have been atiahzed to identify the chatter frequencies and the peak amplitude values. Chatter was found to occur at two domitiant mode frequencies of the spindle. These mode frequencies at which chatter occurred have been compared with the chip serration frequencies in a wide cutting speed range for different conditions of cutting. It has been concluded from these findings that chatter occurs during end milling due to the resonance of the machine tools system component when the frequeticy of primary .serrated teeth formation is approximately equal to the "prominent natural frequency" modes of the system components, which are the two mode frequencies of the VMC machine spitidle in this particular case.