In this paper, the waveforms measured by the strain gauge in the tapping test on a number of healthy subjects and patients with Parkinson's disease are analyzed with the objective of reaching a quantitatively evaluation of the associated symptom. It has been observed that the waveform of a patient with Parkinson's disease becomes more irregular as the symptom is getting more serious, while the waveform of a healthy subject is rather regular. In this study, the regularity of the waveform is evaluated by the so-called trajectory parallel measure. The results show a large difference in the trajectory parallel measure of the waveforms of healthy subjects vs. those of the Parkinson's disease patients. Furthermore, the trajectory parallel measure of Parkinson's disease patients can be quantitatively ranked to correlate to the degree of the symptom. This paper begins with a brief description about Parkinson's disease. The trajectory parallel measure is then introduced and applied to analysis of the waveforms of both healthy subjects and patients with Parkinson's disease. Illustrative results are shown to demonstrate the applicability of the proposed analysis methodology.