Pulse transit time (PTT) is important in the study of arterial viscoelastic properties, wave reflections and sympathetic activities. PTT is usually defined as the time interval between the R-wave in the electrocardiogram (ECG) and a characteristic point in the photoplethysmogram. However, it was found that photoplethysmographic signals are affected by the contacting force between the photoplethysmographic sensor and the measurement site. Therefore, the aim of this study is to examine the effect of contacting force on PTT. Four characteristic points in photoplethysmogram were selected to define PTT: (1) the point at 90% of the pulse amplitude (PTT1), (2) the point at 10% of the pulse amplitude (PTT2), (3) the peak of the first derivative of photoplethysmogram (PTT3), and (4) the peak of the second derivative of photoplethysmogram (PTT4). Fifteen healthy subjects participated in the experiment and ECG and photoplethysmographic signals were recorded under different contacting forces ranging from 0.1 N to 0.5 N. It was found that the PTT defined by the point near the foot (PTT2 and PTT4) or on the rising limb (PTT3) of photoplethysmogram increased with the contacting force till near the zero transmural pressure, after that PTT kept at an almost constant level. Whereas such changing trend was not found in PTT defined by the point near the peak of photoplethysmogram